5.6.3error 如下：
An assembly with the same name `UnityEngine.UI' has already been imported. Consider removing one of the references or sign the assembly
answer:I deleted all the c# project files at the root of the folder structure and reloaded the project and the error disappeared. Try that :)
anthor one:Click Assets > Reimport All, that should fix it.
third one:closed and reopened the project and that was enough to fix it,using Unity 5.6.3
or，If for some reason the answer provided by @tomgrin10 doesn't work, you can also close the project, open a file browser to
the project's directory/folder and then delete the "Library" folder and the "Temp" folder (if it exists), then re-open the project and wait for the re-indexing to complete.
I used the last one to solve the problem