为什么git merge在推送合并提交后会生成两个相同的更改历史记录？

I have merged develope branch to master branch, then pushed the origin to remote. As a result there are two same change history and the merged history is the same of the last commit in the develop branch. The merged and commited master branch is always 1 commit ahead than develop branch, Why? At last, if I merge the new commit history on master back to develop then two branch will be same and no new commit history is generated.I have merged develope branch to master branch,