Avoid duplicate gutter markers

To avoid remote, local and merge diff views adding overlapping gutter markers of the same type on the same line. We leave it to CM to handle multiple markers of different types on the same line, but that shouldn't really apply for us.

packages/nbdime/src/common/mergeview.ts

@@ -332,7 +332,18 @@ const gutterMarkerField = StateField.define<RangeSet<MergeMarker>>({
               : e.value.block
               ? conflictBlockMarker
               : conflictMarker;
-          gutters = gutters.update({ add: [marker.range(e.value.from)] });
+          // check for overlap (duplicates) with same type
+          let overlap = false;
+          gutters.between(e.value.from, e.value.from, (from, to, value) => {
+            if (from === e.value.from && value.eq(marker)) {
+              overlap = true;
+              return false;
+            }
+            return;
+          });
+          if (!overlap) {
+            gutters = gutters.update({ add: [marker.range(e.value.from)] });
+          }
         }
       }
       if (e.is(removeGutterMarkerEffect)) {