Add note about existing github issues

CONTRIBUTING.md

@@ -46,6 +46,23 @@ The GitHub issue tracker is for *bug reports* and *features requests* for the
 Vimspector project, and on-topic comments and follow-ups to them. It is not for
 general discussion, general support or for any other purpose.
 
+Please **search the issue tracker for similar issues** before creating a new
+one. There's no point in duplication; if an existing open issue addresses your
+problem, please comment there instead of creating a duplicate. However, if the
+issue you found is **closed as resolved** (e.g. with a PR or the original user's
+problem was resolved), raise a **new issue**, because you've found a new
+problem. Reference the original issue if you think that's useful information.
+
+Closed issues which have been inactive for 60 days will be locked, this helps to
+keep discussions focussed. If you believe you are still experiencing an issue
+which has been closed, please raise a new issue, completing the issue template.
+
+If you do find a similar _open_ issue, **don't just post 'me too' or similar**
+responses. This almost never helps resolve the issue, and just causes noise for
+the maintainers. Only post if it will aid the maintainers in solving the issue;
+if there are existing diagnostics requested in the thread, perform
+them and post the results.
+
 Please do not be offended if your Issue or comment is closed or hidden, for any
 of the following reasons:
 
@@ -53,6 +70,7 @@ of the following reasons:
 * The issue or comment is off-topic
 * The issue does not represent a Vimspector bug or feature request
 * The issue cannot be reasonably reproduced using the minimal vimrc
+* The issue is a duplicate of an existing issue
 * etc.
 
 Issue titles are important. It's not usually helpful to write a title like