Two identical transactions and hashes in Block Hashing algorithm As I was reading Bitcoin hashing algorithm, I see that a Block Header contains hashes in the form of fields like 
hashPrevBlock , hashMerkleRoot . I have following questions:

1) Are the transactions 100% secure? If yes, then I am wondering if Hash is the reason behind it? Since moving forward in a hash is
easy than backwards.

2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated
based on the Hashes ? 

Please elaborate.
1) Are the transactions 100% secure?


No.


  2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated based on the Hashes?


Yes. It's difficult, though, because each transaction must include the hash of the previous transaction, and so on.

It used to be much easier. A while back, it was pointed out that two coinbases could have identical hashes, and so create identical transactions down the line. See also: Why block height is required in coinbase?