The short answer is that each time you Paste a Memorized citation, you get a new and standalone citation. Period. If you Paste it ten times, then you have eleven standalone citations - the original plus the ten times you pasted it.
. . .
Is it possible that you made a change in the Source Details that didn't actually change the footnote sentence?
I agree with Jerry.
As I've mentioned a number of times, my Master Sources are almost all based on the Book, Basic Format template. I'll attach a screen cap of my 1940 census Master Source, showing how it becomes a Source Citation. The most important thing to remember may be that anything on a YELLOW portion of the dialog (including the Master Text tab) is the Master Source. Anything on a GREEN portion of the dialog (including the Detail Text tab) forms the Source Citation. Notice that my brief Source Details are part of the footnote sentence, so any changes there will cause a Source Index to show that there are multiple (similar, but not identical) citations. Changes made to the Detail Text tab are not reflected in the footnote sentence, and so will not create multiple entries in the Source Index.