Upgrading to Wix Toolset v6 Causes Build Exception #9028

WiX Version

6.0.0

.NET or MSBuild or Visual Studio Version

.NET 9.0.200

HeatWave Version

1.0.5.9

Windows Version

Win11 24H2

Repro Repo

https://github.com/oollie34/TmpWixBug

Repro Steps

Build the "FakeService" repo I have recreated which reproduces this problem.
The fairly simple installer works with no issue on Wix v5, but as soon as it is migrated to Wix v6 the following error appears:

3>wix.exe : error WIX0001: System.InvalidOperationException: Sequence contains more than one matching element

There is no further information, so I cannot source which file / call is causing this.

Actual Result

The exception in the output of wix.exe: System.InvalidOperationException: Sequence contains more than one matching element

Expected Result

A continued build.

Acknowledgements

• I acknowledge that this is a fully completed bug report. It is not a question or attempt to get help debugging my issue (because those should be sent to Discussions).