What may have happened is your computer may have multiplied the command if you did more than strike the key (i.e., if you held it down). SOME keyboard shortcuts only activate once per key press, but paste commands repeat if you hold the key down...

Hopefully, this may help as an alternate, using only the mouse:

Start by left-clicking and holding next to the text you wish to copy. Drag the mouse until the entire passage you wish to copy is highlighted. Once the correct area is highlighted, let go of the button. DO NOT PRESS ANYTHING ELSE, as it will undo what you've done.

RIGHT click on the highlighted section - do not hold. This should bring up a short menu of commands, including "cut", "copy", "paste" and "delete". You want copy. 'Cut' will work too, but if it's from a document that you're working on, it will delete the passage from there and let you paste the piece elsewhere. Copy leaves the original intact.

Go to the document (or screen, or what have you) that you wish to paste the text to. Position the mouse where you'd like it pasted, then RIGHT click. This will bring up the same drop down menu as before - select PASTE.

This should accomplish the same thing as what you were doing, but mostly eliminates the chance of keyboard stutter, as with the mouse it only accepts the command once.

I hope this helps.

SeanM