SI-5675 Discard duplicate feature warnings at a position

When -feature has not been enabled, we were double counting
identical feature warnings that were emitted at the same position.
Normal error reporting only reports the first time a warning
appears at a position; feature warning counter incrementing
should behave the same way.

@hubertp: Fixed .check files that were broken in the original commit.

Author: retronym

test/files/jvm/interpreter.check

@@ -97,7 +97,7 @@ scala> case class Bar(n: Int)
 defined class Bar
 
 scala> implicit def foo2bar(foo: Foo) = Bar(foo.n)
-warning: there were 1 feature warnings; re-run with -feature for details
+warning: there were 1 feature warning(s); re-run with -feature for details
 foo2bar: (foo: Foo)Bar
 
 scala> val bar: Bar = Foo(3)
@@ -271,7 +271,7 @@ scala> xs map (x => x)
 res6: Array[_] = Array(1, 2)
 
 scala> xs map (x => (x, x))
-warning: there were 1 feature warnings; re-run with -feature for details
+warning: there were 1 feature warning(s); re-run with -feature for details
 res7: Array[(_$1, _$1)] forSome { type _$1 } = Array((1,1), (2,2))
 
 scala>