Reproduction steps:
1. Open the attached project (DuplicateEncoderCrash.zip).
2. Build and run for iOS.

Actual behavior:
- Application crashes after the splash screen.

Reproduced with:
Metal
2017.2.0p4 2017.3.0f1, 2018.1.0a7

Not reproducible with:
GLES
or
2017.1.2p3

Notes:
- May be related to case 969730 .
- Full callstack attached in the edit.

* thread #15, name = 'UnityGfxDeviceWorker', stop reason = signal SIGABRT
frame #0: 0x0000000181321348 libsystem_kernel.dylib`__pthread_kill + 8
frame #1: 0x0000000181435354 libsystem_pthread.dylib`pthread_kill$VARIANT$mp + 396
frame #2: 0x0000000181290fd8 libsystem_c.dylib`abort + 140
frame #3: 0x0000000181264a5c libsystem_c.dylib`__assert_rtn + 220
frame #4: 0x00000001835114e0 Metal`MTLReportFailure + 764
frame #5: 0x00000001a0b3fcc4 AGXMetalA9
frame #6: 0x00000001a5339698 MetalTools`-[MTLDebugCommandBuffer renderCommandEncoderWithDescriptor:] + 396
* frame #7: 0x000000010110069c utktest`::CreateNewEncoder() at MetalContext.mm:115 [opt]
frame #8: 0x00000001010f21fc utktest`::SetupMetalFBO() at FrameBufferMetal.mm:386 [opt]
frame #9: 0x00000001010f2338 utktest`::PrepareRenderTargetIfNeeded() at FrameBufferMetal.mm:416 [opt]
...