How often can one have a repeat cortisone injection (it worked) to release a trigger finger (the rheumatologist says trigger, the gp had thought dup's?

Every 3-4 months: Usually steroid injections are given only every 3-4 months.

A locking finger is: more commonly a trigger finger, from the tendon below. One may have a dupuytens nodule or cord and think that it is the cause but that is highly unlikely. Duuytrens disease acts in a more superficial plane than the tendon. Often I see patients mis-informed or they have both issues, Dupuytrens and trigger finger. Injection can be curative fairly often OR last for a few weeks or a few months.