Well, the experiment has come to an end, and it's time to report back. I never got round the duplication issue, and ended
up with several contacts / appointments duplicated numerous times, which proved a real pain.
The key seems to be not to have both ActiveSync and Plaxo sync'ing with more than one machine. The following steps should help you keep everything in order, without duplication.
1. Update Plaxo.
2. Clear all PIM from computers, delete ActiveSync partnerships, uninstall Plaxo.
3. Reinstall Plaxo on a single 'master' computer, and sync data.
4. Attach Smartphone, create a new partnership, and Sync data to phone.
5. Create further partnership with second computer, but do NOT install Plaxo, and sync from phone.
I don't know what you do if you want more than two computers in sync, as ActiveSync is limited to two partnerships (I believe.)
An alternative might be to synchronize the computers using Plaxo, and then only create a single ActiveSync partnership.