Insert PDF page into PDF page collection. After this action PDF page collection contains 2 references to the same PDF page, not 2 different PDF pages.
Remove PDF page from PDF page collection. During this action the first reference to PDF page is removed from PDF page collection, i.e. you are removing PDF page which you have added before.
Here is the first possible way for solving your task:
Get reference to PDF page.
Insert PDF page into PDF page collection - you can do this because you have reference to PDF page.
Remove PDF page from PDF page collection using the RemoveAt method (not using Remove method).
Here is the second possible way for solving your task:
Get reference to PDF page.
Remove PDF page from PDF page collection.
Insert PDF page into PDF page collection - you can do this because you have reference to PDF page.