on behalf form + user account creation attempts to create user for org

I ran across this on 4.7.9, upgraded to 4.7.10 and it persists. I was testing on WordPress. To reproduce:

• create a contrib page using on behalf of
• for the individual's profile, select one that has user account creation enabled
• visit the form in a private window (i.e. not logged in) so the user account creation fields are exposed. complete the form.

The form will error with duplicate entry on UI_uf_name_domain_id – i.e. it's erroring when creating the uf_match record.

The underlying problem is that it's creating a uf_match record between the individual and the user account AND the org and the user account. Only one user account is being constructed, but the uf_match creation is triggered twice.

This effectively renders user account creation impossible when also using an on-behalf-of form.