Betterfix : The system should look too if it have different tile enitiy, not only at the block id and the block damage.
If i have a command block with:

/setblock ~ ~2 ~ 68 3 replace {Text1: Hello!}

and i power it, it will place a sign . if i have in the same position a command block with

/setblock ~ ~2 ~ 68 3 replace {Text1:Hello2!}

it will doesn't place the sign with the text "Hello2!". That's a bug ?