D'oh! (bashes head against wall).
I've now deleted it, I'll wait and see if the problem comes back. Incidentally the CPD I deleted was 1500kB, the newly-created CPD 900kB.
Update. The problem did indeed come back, when I edit a topic containing a snippet RH sometimes hangs for a while (several minutes) and eventually crashes.
While it is hanging it is using 50% CPU (ie all of one core) and the memory allocated to RH increases steadily to about 1.3 MiB. Presumably it crashes when it can't get any more memory.
This is RH 8.0.2, XP Pro SP3, 4GiB RAM.
Anyway I removed the snippets and since then have had no crashes. Renaming the CPD helped by keeping the system stable for a couple of editing rounds -- that was enough to remove the snippets.
01-16-15. RoboHelp 10.
The attempt to insert a copied snippet into expanding text box (part of an existing bulleted paragraph in a table) led to an immediate crash.
Tried importing snippet (one word with embedded expanding text) into existing bulleted paragraph in table and that, too, resulted in an immediate crash.
I don't plan to try that again.