Multiple apps with the same URL Scheme - iOS
If two apps register the same custom URL scheme, it is undefined which app will actually be launched. One will be launched but there is no way to know.
Here's a real kicker. If you have two apps on your device with the same URL scheme, and you delete the one that actually gets launched, the other one will not get launched by the URL without rebooting the iOS device.
Your best solution is to ensure your custom URL scheme is not trivial so there is little chance another app will have the same scheme.
According to the Apple docs:
If multiple third-party applications register to handle the same URL scheme, it is undefined as to which of the applications is picked to handle URLs of that type.
Exact quote from the Apple docs, for 8 consecutive years:
Note: If more than one third-party app registers to handle the same URL scheme, there is currently no process for determining which app will be given that scheme.
Sources:
- Communicating with Other Apps (archive from Dec 2011)
- Implementing Custom URL Schemes (archive from May 2019)
As of September 2019, this part of the documentation was removed from the Apple website. The closest related document may now be Register Your URL Scheme.
Updated for 2016
Apple's policy has changed since 2012. Today they indicate that multiple apps will not be allowed to register for the same scheme. Registration time is a much cleaner place to address contention than runtime, since it avoids the security issues discussed it other answers.
Note: If more than one third-party app registers to handle the same URL scheme, there is currently no process for determining which app will be given that scheme.