The results of merge code are inconsistent with expectations

Hi, there is a strange result after merging code. the operations is as following:

  1. create a branch from master named ‘release_test’, which just contains one userDO2VONew method in
  2. the branch ‘develop’ also just contains one ‘userDO2VONew’.
  3. merge the ‘develop’ branch to ‘release_test’ branch
  4. but the result of contains two userDO2VONew methods.

the version of gitlab is GitLab Community Edition [12.3.5]
what should i check first for this problem?
many thanks in advance!