How to reproduce:
1. Open attached Unity project "DeleteME"
2. Open scene "Vector4AnimationIssue"
3. Select GameObject-->Quad in the Hierarchy window
4. Expand Vector4Mat material
5. Open Animation window
6. Remove property in the AnimateVector4 animation
7. Add Quad-->Mesh Renderer--> Material._Vector 4 property
8. Notice 4 properties: x, y, z and w
9. Turn on recording in the Animation window
10. Change on of x, y, z or w values of the Vector4Mat material in the Inspector window
11. Notice 4 more properties: r, g, b and a

Expected result: 4 properties are added to the animation.
Actual result: 8 properties are added to the animation. Values are duplicated between xyzw and rgba.

Reproducible with: 2018.1.0b5, 2017.3.1f1, 2017.2.1p3, 2017.1.3p1, 5.6.5p1.