To reproduce:
1. Open attached project "2018.1.9f2.zip"
2. Set Graphics API to OpenGLES3
3. Open "test.unity" scene
4. Open "testshader.shader" change something and save
5. Observe an error in Editor console:
GLSL compilation failed:
ERROR: 0:15: 'b1_Type' : syntax error: syntax error
6. Select "testshader.shader" and in Inspector window change Compiled code drop-down settings to Custom: OpenGLES30 and press Compile and show code
7. Observe as there are duplicate struct defines

Expected result: struct defines do not duplicate
Actual result: the are duplicate struct defines

Reproduced on:
2017.4.0f1 2017.4.17f1 2018.1.9f2 2018.2.19f1 2019.1.0a12

Notes:
On 2018.3.0f1 opening the scene Unity crashes.
Doesn't reproduce on OpenGLES2 and Metal