[skip ci] Add policy for duplicate PRs (#24613)

AGENTS.md

@@ -1,5 +1,12 @@
 # Agents
 
+## Pull requests
+
+Before starting work and again before opening a pull request, check for existing
+pull requests as described in [CONTRIBUTING.md](CONTRIBUTING.md#pull-requests).
+Do not open a duplicate of an existing pull request, unless you have a very good
+reason to do it.
+
 ## Vulnerability reports
 
 When scanning php-src for vulnerabilities, please respect our security policy

CONTRIBUTING.md

@@ -34,6 +34,19 @@ had several contributions accepted, commit privileges are often quickly granted.
 PHP welcomes pull requests to [add tests](#writing-tests), fix bugs and to
 implement RFCs. Please be sure to include tests as appropriate!
 
+Before starting work and again before submitting a pull request, search the
+[existing pull requests](https://github.com/php/php-src/pulls?q=is%3Apr),
+including closed ones, for the same issue or change. Check the bug report for
+linked pull requests, and search by issue number, relevant function names and
+keywords.
+
+If an open pull request already addresses the problem, help test or review it
+to make it better instead of opening an equivalent pull request.
+
+When multiple pull requests provide equivalent fixes of comparable quality,
+we follow a first-come, first-served principle. We prefer the first correct
+pull request.
+
 By submitting a pull request, you certify that you have the necessary rights
 to submit the work, that the work does not violate any third-party rights
 (including those of your employer, if applicable), and that you license your