System identifies two of the same curve. Closes #171

ChangeLog.txt

@@ -2,6 +2,7 @@
 - Z value change for a layout piece.
 - Fix issue with Custom template.
 - Fix issue with tile font size.
+- [smart-pattern/valentina#171] System identifies two of the same curve.
 
 # Valentina 0.7.50 February 14, 2022
 - Fix regression. Minimal seam allowance width is less than the point accuracy values.

src/libs/vtools/dialogs/dialogtoolbox.cpp

@@ -146,7 +146,8 @@ auto DoubleCurve(const VPieceNode &firstNode, const VPieceNode &secondNode, cons
         }
 
         // The same curve, but different modeling objects
-        if (curve1->getIdObject() == curve2->getIdObject())
+        if (curve1->getIdObject() != NULL_ID && curve2->getIdObject() != NULL_ID &&
+                curve1->getIdObject() == curve2->getIdObject())
         {
             error = QObject::tr("Leave only one copy of curve '%1'").arg(curve1->name());
             return true;