It is possible by gpgsm (commandline or via gpgme)
that certificates are doubled. This duplication prevents
other encryption operation, thus rendering the crypto system
unable to send out encrypted email.

Could reproduce this with gpgsm 2.0.4 and 2.0.5.

Werner Koch wrote:

I could not encrypt to a recipient because I had his certificate
twice. How could this happen?

That may be cuased due to a race condition. Checking two signatures at
tghe same time. The keybox is not kept lock between checking for
existence of the certificate and storing it later.

gpgsm: certificate not found: Mehrdeutiger Name

The best solution is to add an additional check on whether the
certificates are identical before issuing this error.

The actual problem will go away if we eventually add an index over the
fingerprint - this would then horw an error if an identical certificate
is added to the keybox.

The race condition seems to occur more often when several recipients are added.