To reproduce:

1. Download attached project “myVegasProject.zip" and open in Unity
2. Change Platform to iOS
3. Right click on Project view and select "Reimport All"
4. When Unity reimports all Assets press Tools > Build Tool for Local
5. Observe that it opens a Custom Build command window
6. In Custom Build window set Environment to STAGE
7. In Custom Build window click "Prepare iOS Editor Environment" and wait for it to complete
8. In Custom Build window click "Build iOS App"
9. Wait for it to try and build the player, the IL2CPP error will appear in the console before the Xcode project is generated.

Expected results: Custom Build window generates an Xcode project
Actual results: During the Build process the IL2CPP Error will appear in the Console before the Xcode project is generated:

IL2CPP error for type 'UnityEngine.UI.GridLayoutGroup/Corner' in assembly '/Users/ryan/Downloads/myVegasProject/unity/Temp/StagingArea/Data/Managed/UnityEngine.UI.dll'
Additional information: Build a development build for more information. Sequence contains more than one matching element
il2cpp.exe didn't catch exception: System.InvalidOperationException: Sequence contains more than one matching element

Notes:
- I can test this issue only on iOS because Custom Build window was created for iOS Platform
- Could not check on Unity 2017.4 and 2018.2 because project becomes corrupted
- If you get Errors such as "DXT5 compressed textures are not supported when publishing to iPhone" when building then Reimport all Assets (make sure platform is set to iOS beforehand) and try again
- There is attached zipped up Temp/StagingArea/Data/Managed folder after receiving the build error to help with debugging this issue

Reproduced on 2018.3.0f2, 2018.3.3f1, 2019.1.0b1 and 2019.2.0a3