block hash and list since block? I am writing transaction monitoring service but I am having some problems with listing since block hash. I have bitcoind daemon and when I execute listTransactions some transactions have same block hash so if I would try to listSinceBlock I would have problems. How this all works?
The block hash identifies a block. A block can contain multiple transactions, so listsinceblock lists all transactions in blocks newer than the given block hash.

So f.e., if you have this block structure:

block 5 hash: qwertyyuiop
|   tx100,tx101,tx102,tx103
v
block 6 hash: asdfghjklkl
|   tx104,tx105,tx106,tx107,tx108
v
block 7 hash: zxcvbnmnbvc
    tx109,tx110,tx111


When you would do listsinceblock qwertyyuiop, you would get all transactions in blocks older than block 5, so tx104 all the way up to tx111.