[Kinetics] Prevent double counting in reaction path diagrams

This fixes the double counting that occurs in reactions like:

    H + HO2 => 2 OH

src/kinetics/ReactionPath.cpp

@@ -8,6 +8,7 @@
 
 #include "cantera/kinetics/ReactionPath.h"
 #include "cantera/kinetics/reaction_defs.h"
+#include <set>
 
 using namespace std;
 
@@ -718,13 +719,22 @@ int ReactionPathBuilder::build(Kinetics& s, const string& element,
         if (m_elatoms(m, i) > 0) {
             size_t nr = m_reac[i].size();
             size_t np = m_prod[i].size();
+            // each A->B species pair should only be counted once per reaction,
+            // to avoid double-counting fluxes involving species with non-unity
+            // stoichiometric coefficients
+            std::set<std::pair<size_t, size_t>> counted_pairs;
 
             for (size_t kr = 0; kr < nr; kr++) {
                 size_t kkr = m_reac[i][kr];
                 string fwdlabel = reactionLabel(i, kr, nr, m_reac[i], s);
 
                 for (size_t kp = 0; kp < np; kp++) {
                     size_t kkp = m_prod[i][kp];
+                    if (counted_pairs.find({kkr,kkp}) == counted_pairs.end()) {
+                        counted_pairs.insert({kkr,kkp});
+                    } else {
+                        continue;
+                    }
                     string revlabel = "";
                     for (size_t j = 0; j < np; j++) {
                         if (j != kp) {