importing the samemixin template results in isn't a template error instead of the expected conflicts with alias error

[dlangBugzillaToGithub]

or reported this on 2017-09-26T13:07:03Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17859

Description

a.d:
-----------------
module a;

mixin template A(T) {
	struct A {
		T t;
	}
}

struct X {
	int a;
}
-----------------

b.d: 
-----------------
module b;
import a: A;
import a: A;
mixin A!int;

import a: X;
import a: X;
X x;
-----------------

running `dmd -o- a.d b.d`:
                                                                                                                                                      
b.d(4): Error: mixin b.A!int A isn't a template
b.d(7): Error: alias b.X conflicts with alias b.X at b.d(6)

I would expect the error for `A` will be the same error I get on `X`