Copy and Paste Keyframes

It is better for you to be as descriptive as possible.

If I had to guess I would guess you are trying to copy and paste keyframes across more than one layer.   You cannot do this without a script.  You can only CTRL+C keyframes from a single layer.  Once copied you can paste them to as many layers as you want (at least with basic transform properties) but the original selection can only be from a single layer.  If you select key frames from mulitple layers at once and copy paste After Effects will duplicate the layers, as you've discovered.

As for your claim that you "do it all the time" and it's always worked?  I dare say you are mistaken and that you're doing something different then you've always done.

Impossible to know. You have not provided any system information, no version information, no details about which properties you are trying to manipulate. On a hunch one could assume that you are messing wuth the new Master Properties, where this is a known limitation, but who's to say? You need to be much more precise and offer up better information.

Mylenium