Instead of creating an exact copy of existing constraint, select it.

CHANGELOG.md

@@ -35,6 +35,8 @@ New constraint features:
     in the text window.
   * Distance constraint labels can now be formatted to use SI prefixes.
     Values are edited in the configured unit regardless of label format.
+  * When creating a constraint, if an exactly identical constraint already
+    exists, it is now selected instead of adding a redundant constraint.
   * It is now possible to turn off automatic creation of horizontal/vertical
     constraints on line segments.
   * Automatic creation of constraints no longer happens if the constraint

src/constraint.cpp

@@ -745,6 +745,17 @@ void Constraint::MenuConstrain(Command id) {
         default: ssassert(false, "Unexpected menu ID");
     }
 
+    for(const Constraint &cc : SK.constraint) {
+        if(c.h.v != cc.h.v && c.Equals(cc)) {
+            // Oops, we already have this exact constraint. Remove the one we just added.
+            SK.constraint.RemoveById(c.h);
+            SS.GW.ClearSelection();
+            // And now select the old one, to give feedback.
+            SS.GW.MakeSelected(cc.h);
+            return;
+        }
+    }
+
     if(SK.constraint.FindByIdNoOops(c.h)) {
         Constraint *constraint = SK.GetConstraint(c.h);
         if(SS.TestRankForGroup(c.group) == SolveResult::REDUNDANT_OKAY &&
@@ -755,7 +766,6 @@ void Constraint::MenuConstrain(Command id) {
     }
 
     SS.GW.ClearSelection();
-    SS.GW.Invalidate();
 }
 
 #endif /* ! LIBRARY */

src/sketch.h

@@ -655,6 +655,15 @@ class ConstraintBase {
     bool        reference;  // a ref dimension, that generates no eqs
     std::string comment;    // since comments are represented as constraints
 
+    bool Equals(const ConstraintBase &c) const {
+        return type == c.type && group.v == c.group.v && workplane.v == c.workplane.v &&
+            valA == c.valA && valP.v == c.valP.v && ptA.v == c.ptA.v && ptB.v == c.ptB.v &&
+            entityA.v == c.entityA.v && entityB.v == c.entityB.v &&
+            entityC.v == c.entityC.v && entityD.v == c.entityD.v &&
+            other == c.other && other2 == c.other2 && reference == c.reference &&
+            comment == c.comment;
+    }
+
     bool HasLabel() const;
 
     void Generate(IdList<Param, hParam> *param);