When i trying to mine with small hashrate, i got this errors:
The "Block stale detected" message is normal (though eliminated in jtoomimnet) and does not indicate a problem.
The "submitted share more than once" message is more unusual, but by itself does not itself indicate that things aren't working, nor does it give much of a hint how things would be failing if they were failing. My only guess for why you might be getting that is if the difficulty set by your node is too low. It's also possible that your mining hardware is just buggy or your network is buggy/congested/laggy or your node's CPU is getting saturated.
And also i don't know if jtoomimnet has a block finding reward like p2pool.org has!
It currently has the same 0.5% bonus that mainnet has. I might want to increase that in the future, but for now it's the same.
Is the forrestvnet not well configured
or is just a big blow of bad luck?
With mainnet's current hashrate of 760 Th/s, it is expected to find one block every 95 days or so. Mainnet last found a block 85 days ago on August 15, 2017, so this doesn't even qualify as bad luck. With this hashrate, "bad luck" would mean not finding a block for about a year.