Q:

Github allowing one's changes to replace from feature branch

As we are using gitHub as a licence for our project.  we are getting code issues and because of this we got fired from our clients. 
Let me explain........
We are creating a feature branch from master branch , lets called it "X" & "Y".  Now "X" completed work before "Y" and merged code into master.  Now "Y" comes and says I want to merge because they also completed the development but "Y" don't have changes which "X" did so when "Y" merges code into master , github allowing "Y" to wipe out "X" changes without giving any warning or conflict. 
Is that a github feature or bug ? 

A:

First of all, this seems highly unlikely to happen unless there were conflicts on Github and whoever worked on Y rebased/merged master and wiped out all of X's changes. 
The teams I work with also follow work on features the way you have described.  We have best practices in place to ensure no one wipes out another team's code.  Some of them are

Tests ensuring no one is introducing regression bugs
Pull requests don't get merged unless they are up to date with the master branch.  We follow a rebase strategy, but you can also use a merge strategy 
If the projects are closely related, they go to an epic branch first where they are thoroughly tested against requirements using automated tools as well as A/B testing before they are merged into master. 

These pretty much ensure that there are fewer merge conflicts and if they do occur, it is easier to resolve. 