The OPENCLOSE flag is intended to be a matter of optimization, not 
correctness.  Another thread using the already-open file shouldn't 
generally be a problem; it will just be a little faster.  Are you seeing 
a more severe issue than this?

(I can think of a scenario where there are two overlapping 
krb5_cc_retrieve_cred_seqs running in different threads, and when the 
first one finishes, it turns the OPENCLOSE flag back on, causing the 
other thread to run less efficiently.  But I still don't see that causing 
a user-visible problem, just an efficiency issue.)