Filter out duplicate positions

In order to avoid that quick fix proposals are shown multiple times, we
can either filter out duplicate positions or filter out duplicate
proposal display strings. The former is not only faster to compute
(because no proposals need to be computed multiple times) it also
doesn't filter out different proposals that have the same display string
(for whatever reason that should happen in future).

Fixes #1002286

org.scala-ide.sdt.core/src/org/scalaide/core/internal/quickassist/QuickAssistHandler.scala

@@ -84,9 +84,22 @@ final class QuickAssistProcessor(input: IEditorInput, id: String) extends IQuick
 
       (iter foldLeft IndexedSeq[AssistLocation]()) {
         case (ps, a: Annotation) if a.isInstanceOf[ScalaEditorAnnotation] || a.isInstanceOf[IJavaAnnotation] =>
-          val pos = model.getPosition(a)
-          if (isInside(ctx.getOffset, pos.offset, pos.offset+pos.length))
-            ps :+ AssistLocation(pos.offset, pos.length, a)
+          val (start, end) = {
+            val p = model.getPosition(a)
+            (p.offset, p.offset+p.length)
+          }
+
+          def isOffsetInsidePos(offset: Int): Boolean =
+            offset == start || offset == end || (offset > start && offset < end)
+
+          def isPosInsideRange(rStart: Int, rEnd: Int): Boolean =
+            start >= rStart && end <= rEnd
+
+          def posNotYetFound =
+            ps.forall(a => !isPosInsideRange(a.offset, a.offset+a.length))
+
+          if (isOffsetInsidePos(ctx.getOffset) && posNotYetFound)
+            ps :+ AssistLocation(start, end-start, a)
           else
             ps
         case (ps, _) =>
@@ -118,9 +131,5 @@ final class QuickAssistProcessor(input: IEditorInput, id: String) extends IQuick
 
   override def canAssist(ctx: IQuickAssistInvocationContext): Boolean = true
   override def canFix(a: Annotation): Boolean = true
-
   override def getErrorMessage(): String = null
-
-  private def isInside(offset: Int, start: Int, end: Int): Boolean =
-    offset == start || offset == end || (offset > start && offset < end)
 }