Compiler gives no warning for redundant definitions, and obscures errors in earlier definitions

[edwardpeters]

Describe the bug
If two definitions by the same name are given in the same module, the compiler gives no errors or warnings; additionally, any errors in the earlier definition are not shown.

To Reproduce
Compile the following file:

module Scratch.ExampleModule exposing (..)

typeErrorTest : Bool -> String
typeErrorTest x = True
     
typeErrorTest : Bool -> Bool
typeErrorTest x = x

With both make and make -f, this successfully compiles with no warnings given.

Expected behavior
An error should be given for the conflicting definitions, and errors from example functions should not be hidden.

Desktop (please complete the following information):

• OS: [e.g. iOS] : OSX
• Browser: Unused
• Version: 2.89.0

Additional context
This has a negative impact on developer experience, as we get very confusing behavior where a function appears to be compiling but neither its errors nor runtime behavior are actually apparent.