Tie the external reaction and buoyancy properties only once

src/models/FGBuoyantForces.cpp

@@ -130,8 +130,12 @@ bool FGBuoyantForces::Load(Element *document)
 
   PostLoad(document, FDMExec);
 
-  if (!NoneDefined) {
+  // Load() is public and appends to the cell list, so it may be called more than
+  // once to add further gas cells. The properties must only be tied the first
+  // time: tying them again fails and logs an error for each one.
+  if (!NoneDefined && !isBound) {
     bind();
+    isBound = true;
   }
 
   return true;

src/models/FGBuoyantForces.h

@@ -178,6 +178,7 @@ class FGBuoyantForces : public FGModel
   FGColumnVector3 vXYZgasCell_arm; // [lbs in]
 
   bool NoneDefined;
+  bool isBound = false;
 
   void bind(void);
 

src/models/FGExternalReactions.cpp

@@ -85,7 +85,13 @@ bool FGExternalReactions::Load(Element* el)
 
   PostLoad(el, FDMExec);
 
-  if (!Forces.empty()) bind();
+  // Load() is public and may be called more than once to add further forces, so
+  // the properties must only be tied the first time. Tying them again fails and
+  // logs an error for each one.
+  if (!Forces.empty() && !isBound) {
+    bind();
+    isBound = true;
+  }
 
   return true;
 }

src/models/FGExternalReactions.h

@@ -169,6 +169,7 @@ class FGExternalReactions : public FGModel
   //unsigned int numForces;
   FGColumnVector3 vTotalForces;
   FGColumnVector3 vTotalMoments;
+  bool isBound = false;
 
   void bind(void);
   void Debug(int from) override;