What i don't understand is why sometimes a member finds a namecoin and a bitcoin block at the same time. If i count right we had 4 such doubles. Is that just coincidence ?
We should find a NMC block EVERYTIME we find a BTC block.
Remember there is only one block header. The prior NMC block hash is put into coinbase transaction of the BTC block.
1) You get a block header to hash from the server.
2) You hash it and check if it matches difficulty for a share.
3) If it does you submit it to the server.
4a) Server checks if it matches difficulty target for BTC
4b) Server checks if it matches difficulty target for NMC
So each share can be:
a) worthless (just used to track work by miners to ensure fair payout)
b) solution for BTC block
c) solution for NMC block
d) solution for both BTC & NMC blocks.
Now right now (and likely in the future) BTC difficulty is HIGHER than NMC. Thus any share which meets BTC difficulty target meets NMC target. Using smaller numbers may make it easier to understand.
Say to solve a block you pick a random number between 1 & 100 for each share.
The current target for a valid BTC share is < 10.
The current target for a valid NMC share is < 40.
a share w/ value of 41 is worthless. (type a above)
a share w/ value of 14 is valid for NMC but invalid for BTC (type c above)
a share w/ value of 9 is valid for both NMC & BTC (type d above)
Now you may be wondering how you get a type B. It would only occur as a timing issue. While any BTC hash meets the target for a NMC hash if the NMC block has changed the miner is working on a block that can never be a valid NMC hash (no matter how low of hash is found). A "perfect pool" (which isn't possible with current miners) would have a way to tell miner the to update the block but still submit work because it might be good for BTC. Luckily this should be relatively rare.