"variable __000002 already defined" after regression command

Whether executed from the command line or from within the sytax file (do-file), I get, for the following command, the error shown:

. xi: mepoisson event6 year##age_cat5_1##male if (parish_new != 3 & parish_new != 4 & parish_new != 7 & parish_new != 11) & year > 2014 , exp(pop_size_) || par_comm:
variable __000002 already defined
r(110);


I am using Stata 16 and I have successfully executed the command using other similar outcomes. Can anyone suggest reason(s) for this error and how it may be corrected or circumvented?

Regards

Novie