For everyone with the same problem:
I was facing the same problem and commented out the two columns in the file “setdefaultsignature.py”:
#if webappsettings['settings']['zarafa']['v1']['contexts']['mail']['signatures']['all'].get(signatureid):
# print 'Default Signature already exists.'
Thereafter, the signatures will be overwritten during execution.
Please note that when executing “example_wrapper.sh” without a given user, all signatures will be overwritten.
I assume no liability for the fact that everything works ;-P
Sorry for digging but it helped me a lot.