diff --git a/CHANGELOG.md b/CHANGELOG.md
index 65ea7704..de85b8e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,6 +44,8 @@ Bugs fixed:
     causes the line length to collapse.
   * Curve-line constraints (in 3d), parallel constraints (in 3d), and
     same orientation constraints are more robust.
+  * Adding some constraints (vertical, midpoint, etc) twice will now error out
+    immediately, instead of later and in a confusing way.
 
 2.3
 ---
diff --git a/src/constraint.cpp b/src/constraint.cpp
index 24634c0b..86073c7b 100644
--- a/src/constraint.cpp
+++ b/src/constraint.cpp
@@ -85,6 +85,7 @@ hConstraint Constraint::AddConstraint(Constraint *c, bool rememberForUndo) {
     SK.constraint.AddAndAssignId(c);
 
     SS.MarkGroupDirty(c->group);
+    SK.GetGroup(c->group)->dofCheckOk = false;
     SS.ScheduleGenerateAll();
     return c->h;
 }
diff --git a/src/generate.cpp b/src/generate.cpp
index 5c466d8d..acf82637 100644
--- a/src/generate.cpp
+++ b/src/generate.cpp
@@ -528,7 +528,13 @@ void SolveSpaceUI::SolveGroup(hGroup hg, bool andFindFree) {
     MarkDraggedParams();
     g->solved.remove.Clear();
     SolveResult how = sys.Solve(g, &(g->solved.dof),
-                           &(g->solved.remove), /*andFindBad=*/true, andFindFree);
+                                   &(g->solved.remove),
+                                   /*andFindBad=*/true,
+                                   /*andFindFree=*/andFindFree,
+                                   /*forceDofCheck=*/!g->dofCheckOk);
+    if(how == SolveResult::OKAY) {
+        g->dofCheckOk = true;
+    }
     g->solved.how = how;
     FreeAllTemporary();
 }
diff --git a/src/sketch.h b/src/sketch.h
index 74f0d3a1..c0ba6822 100644
--- a/src/sketch.h
+++ b/src/sketch.h
@@ -141,6 +141,7 @@ public:
     double      scale;
 
     bool        clean;
+    bool        dofCheckOk;
     hEntity     activeWorkplane;
     double      valA;
     double      valB;
diff --git a/src/solvespace.h b/src/solvespace.h
index 28a653a1..3fd468e0 100644
--- a/src/solvespace.h
+++ b/src/solvespace.h
@@ -444,7 +444,7 @@ public:
     void EvalJacobian();
 
     void WriteEquationsExceptFor(hConstraint hc, Group *g);
-    void FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad);
+    void FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad, bool forceDofCheck);
     void SolveBySubstitution();
 
     bool IsDragged(hParam p);
@@ -452,7 +452,7 @@ public:
     bool NewtonSolve(int tag);
 
     SolveResult Solve(Group *g, int *dof, List<hConstraint> *bad,
-                bool andFindBad, bool andFindFree);
+                      bool andFindBad, bool andFindFree, bool forceDofCheck = false);
 
     void Clear();
 };
diff --git a/src/system.cpp b/src/system.cpp
index 7ff8c3d2..5e6b8070 100644
--- a/src/system.cpp
+++ b/src/system.cpp
@@ -359,7 +359,7 @@ void System::WriteEquationsExceptFor(hConstraint hc, Group *g) {
     g->GenerateEquations(&eq);
 }
 
-void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad) {
+void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad, bool forceDofCheck) {
     int a, i;
 
     for(a = 0; a < 2; a++) {
@@ -382,7 +382,9 @@ void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad) {
 
             // It's a major speedup to solve the easy ones by substitution here,
             // and that doesn't break anything.
-            SolveBySubstitution();
+            if(!forceDofCheck) {
+                SolveBySubstitution();
+            }
 
             WriteJacobian(0);
             EvalJacobian();
@@ -397,7 +399,7 @@ void System::FindWhichToRemoveToFixJacobian(Group *g, List<hConstraint> *bad) {
 }
 
 SolveResult System::Solve(Group *g, int *dof, List<hConstraint> *bad,
-                  bool andFindBad, bool andFindFree)
+                          bool andFindBad, bool andFindFree, bool forceDofCheck)
 {
     WriteEquationsExceptFor(Constraint::NO_CONSTRAINT, g);
 
@@ -418,7 +420,9 @@ SolveResult System::Solve(Group *g, int *dof, List<hConstraint> *bad,
     param.ClearTags();
     eq.ClearTags();
 
-    SolveBySubstitution();
+    if(!forceDofCheck) {
+        SolveBySubstitution();
+    }
 
     // Before solving the big system, see if we can find any equations that
     // are soluble alone. This can be a huge speedup. We don't know whether
@@ -465,7 +469,7 @@ SolveResult System::Solve(Group *g, int *dof, List<hConstraint> *bad,
     rankOk = TestRank();
     if(!rankOk) {
         if(!g->allowRedundant) {
-            if(andFindBad) FindWhichToRemoveToFixJacobian(g, bad);
+            if(andFindBad) FindWhichToRemoveToFixJacobian(g, bad, forceDofCheck);
         }
     } else {
         // This is not the full Jacobian, but any substitutions or single-eq