Q:

Merge different issues on github

Sometimes the developers open new issues regarding the same subject.  
It will be great to have the possibility to merge the issues. 
If it is possible in github, how can I do it? 
If not, what is the best way to link different issues? 

A:

Currently, the simplest way to resolve this situation is to pick one of the issues to be the main one (whether the best documented, oldest, or whatever).  Then, add a comment to the other issues linking to the main issue and close the duplicates. 
Here's an example. 
Here's another showing the "Duplicate of #123" syntax. 
Alternatively, you can leave the issues open and reference them all together in your commit messages: Example. 
I imagine that in the future, the team behind GitHub will make this process easier.