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Inheriting forms duplicates controls into designer

I have a problem with inheriting in Windows Forms.
I created a base form class with some controls and events and a class which inherits from the base class. When I try to open the designer of the inherited class it gives me this error message: "Duplicate component name .... Component names must be unique and case-insensitive".
So this error message appears because it copied some (not all) of the controls of the base form into the designer.cs of the inherited form. All of my controls are 'protected'.
Why it copies some of the controls in general and why not all?

Why does VS2017 keep losing my derived controls?

In my app namespace = DRT, I'm creating control classes (e.g., button, textbox) that derive fron their corresponding Windows control classes, e.g.,
internal abstract class DRT_Button_Abstract : Button
{
....
}
internal class DRT_Button_CancelSearch : DRT_Button_Abstract
{
....
}
internal class DRT_Button_StartSearch : DRT_Button_Abstract
{
....
}
All together I currently have 13 derived classes that derive either from one of my abstracts or from a Windows control class. After a successful build, I see my control classes (e.g., DRT_Button_CancelSearch and DRT_Button_StartSearch) on the Toolbox and I successfully drop them onto my main form. All is fine for a while, but ultimately, I'll go to open the main form.cs [Design] (i.e., the UI designer) and it will show the error The variable '{control property name}' is either undeclared or was never assigned. for some combination of my controls.
When I examine the main form Designer.cs file, the expected code for all the controls is present EXCEPT for the expected new statement. They are not present in the main form Designer.cs file. For example, I expect to see this.drt_Button_CancelSearch = new DRT.DRT_Button_CancelSearch(); but its missing
I've tried ignoring the error, proceeding to the UI designer windows to re-apply the lost controls, but the problem just repeats with the newly applied control
What the heck is going on? Is there a way to recover from this situation?

This is most likely a problem of the Designer not being able to clear/reload its cache. There is not much you can do. In the past I:
closed and reopened all designers that have user controls
put all the controls in a separate project (in the same solution)
put all the controls in a separate solution/Visual Studio instance and set a proper reference to the controls' dll (or even nuget package)
With the first two options I have had varying success. Reopening the designer is not very convenient and doesn't work.
That last option is the best but also the most annoying because every adjustment requires a rebuild of the project and update of the reference/package.
Also make sure that all controls that you create have public default constructors and function well when this constructor is used.

Can a Form be used as a user control?

I need to make a UserControl that can be used for multiple projects. But it needs to be a Form so the user can just add a reference to the library and call the form.
I've seen third party companies like Telerik and DevExpress use custom forms that can be added to a project.
How would I accomplish this? I've been looking through SO and various posts from Google, but have not been successful in my searches.
EDIT I was assuming it had to be a UserControl for some reason. But it doesn't. I took the suggestion of just adding a form and calling it from that namespace. Works exactly as needed. Thanks everyone.

Just create the form in your library, make it public, and you can call it from anywhere.
Methods to create and call form are:
YourFormClassName FormForUser = new YourFormClassName();
FormForUser.Show();
FormForUser.ShowDialog();

Maybe I don't understand. If I do, then it's straight forward.
Add a project (ProjectWithReusedForm) to your solution that contains the form to be reused.
Add a reference to ProjectWithReusedForm in the second project where you want to use the form
Add a 'using ProjectWithReusedFormNamespace' to the code where you want to use the form
You then can add the statement ReusedForm myForm = new ReusedForm();

You can create BaseForm (either add it into a project directly by adding .cs file or reference something compiled - class library to example). Then just add a new form to a project
namespace MySolution
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
}
}
and chang Form to BaseForm
public partial class Form1 : BaseForm

Just Create form with all controls. and create empty user control
Ex:
do this code inside usercontrol constructor after initialize function
dim obja as new RegForm()
obja.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None
obja.Visible = true
obja.Dock = System.Windows.Forms.DockStyle.Full
me.Controls.Add(obja)

You have to be careful here. Your tag lists winforms, so I am assuming you are using .net and UserControls. Winforms only allows a single form per page. You can add multiple UserControls to a page. If you go with the base form route, the programmer will have to add everything else to your base page. UserControls will offer a little more flexibility in that they can be added to an existing page.

c# how to make visual studio designer show base classes components aswell

I have a class
public abstract class BaseFormClass : UserControl
and another class:
public class DerivedFormClass : BaseFormClass
if i open design view on baseformclass it show 3 components at the top.
but if i open derivedformclass, it does NOT show these 3 components, I have tried declaring
InitializeComponent virtual and overriding, but this made no difference.
Its annoying as if i run it then it displays fine, but i want it to work in designer so it easier to work with in future.
any ideas?

I have found out what works,
'both' class definitions need to extend the base class
so designer and general
public partial class BaseFormClass : UserControl
...
partial class DerivedFormClass: BaseFormClass
...
then the DerivedFormClass's designer will show the components from the BaseFormClass,
an alternative is to just define the baseformclass and then user that control in teh derived form class,
but this depends on what you need to implement.

Multiple classes in one form

I'm using a TabControl in my form and it made me wonder. Right now I have only two tabs and I store procedures relating to both tabs (button handlers, &c.) in the code for the main form, so it looks like this:
public partial class MainForm : Form
{
// ----------TAB1-----------
tab1SearchButtonCLick() {...}
tab1AddButtonCLick() {...}
// ----------TAB2-----------
tab2EditButtonCLick() {...}
tab2SearchButtonCLick() {...}
tab2ClearButtonCLick() {...}
}
That's not a problem now with so little code, but it might be one in the future. Is this an acceptable way of doing this? What's the alternative? I believe I could put the tabs in their own classes, but I'm not sure how I'm going to do that exactly (there's lots of controls in each tab that I'd have to pass as arguments to constructors).

You should move the contents of each tab to a separate UserControl.
Each UserControl should be a self-contained unit that gets whatever data in needs from the main form and fires events to tell the main form to do things.

The tabs have event for a reason - the tab raises the event and doesn't want to handle it.
When you use the tabs (or other objects with events) you need to write the code that handles the event.
If you really want, you can write the class in separate files so you can keep your own logic in separate files.
If you'll notice a Form is generated as a partial class by Visual studio.
That's because the design code is generated in a separate file.
You can do that yourself by declaring other parts of the class as partial in other files.
More about partial in this link