Is it legal to use non-terminal twice in Backus-Naur grammar It's a school assignment:
The problem is:
Write a BN grammar for +67.

The given solution I have been given is following:

&lt;digit&gt;  ::= 6 | 7
&lt;number&gt; ::= &lt;digit&gt; | &lt;digit&gt; &lt;number&gt;
&lt;+67&gt;    ::= + &lt;number&gt;


My question is, when given this simple task, could it be derived like this?

&lt;digit&gt;  ::= 6 | 7
&lt;number&gt; ::= &lt;digit&gt; &lt;digit&gt; 
&lt;+67&gt;    ::= + &lt;number&gt;


More precisely, can I use same non-terminal twice in same rule?
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.