Error inserting row on unique key

I have a large table which uses a sequence for column which is defined as the primary key.
Our systems is a real time data acquisition system, so we have "insert" statements going through it wrapped inside stored procedure.

Every once in a while I get an inserio error on a violation of a unique constraint on the primary key.
Could this be caused by a subsequent call to the stored procedure which took place before the sequence on the prior stored procedure fired?
If so, do you recommend taking the call to the sequence out of the columns constraint definition and manually calling it as the firs step in the SP call?

As I said, this does not happen very often, but I have seen it happen.

Regards,

Benjamin