Clarification regarding how duplicate reactions are handled

[H V]

Hello, 

I was wondering how duplciate reactions are handled / what the differences are in three different cases

1. if the file has both A + B <-> C + D and C + D <-> A + B, how is the effective rate computed? The documentation says the rates are summed, but does it take into account that the duplicate equation gives the backward rate while the firs one has the rate expression for the forward rate. 

2. What is the difference between having these two reactions declared as duplicates or not? A + B -> C + D and C + D -> A + B. 

Thanks!