show: fix duplicate typealias printing bug

If you had 2 aliases that both matched, we might print
```S{T} where T (alias for S{T} where T)```
which is clearly unnecessary.

base/show.jl

@@ -751,7 +751,7 @@ function show(io::IO, ::MIME"text/plain", @nospecialize(x::Type))
     show(io, x)
     if !print_without_params(x) && get(io, :compact, true)
         properx = makeproper(io, x)
-        if make_typealias(properx) !== nothing || x <: make_typealiases(properx)[2]
+        if make_typealias(properx) !== nothing || (unwrap_unionall(x) isa Union && x <: make_typealiases(properx)[2])
             print(io, " (alias for ")
             show(IOContext(io, :compact => false), x)
             print(io, ")")