My clipboard has bifurcated into two clipboards. The normal system clipboard and a mathematica artifact that can't be changed or deleted. The normal one follows the normal rules within mathematica, including all keyboard shortcuts and built-in functions. It can be used to copy and paste to and from mathematica and any other program. However, pasting in mathematica pastes not only the normal clipboard line but also a second line from a previous clipboard that has become stuck somewhere in some other part of mathematica I can't find or access. So you get e.g.

Paste[]

~ artefact

~ a

CopyToClipboard[b]

Paste[]

~ artefact

~ b