Is it legal to use non-terminal twice in Backus-Naur grammar
Yes, you are allowed to use the same non-terminal more than once in a rule.

Note that the two grammars you give do not express the same set of strings. 
The first grammar gives any sequence of 6 or 7 following the +, as  can be applied recursively. The second defines exactly four strings +66, +67. +76, +77.