document data changed during process

Hi all

I have a strange problem
we have a process with a receive step. and it subscribe to a document containing a data record.

when user do operation in process, we found sometimes the data is changed from 1 record to another.
for example:
if current document data is “aaa,bbb”, when a user open a task, it becomes “ccc,ddd”.
it seems the current record is replaced by another record.

does anyone can help on this problem?

thanks in advance