`next` is called twice

[Tommaso Allevi]

Look at the code, when happen this error

fastify-mongodb/index.js

Lines 49 to 51 in ffe4ace

 if (fastify.mongo) { 
   next(new Error('fastify-mongo has already registered')) 
 } else { 

this line is called anyway

fastify-mongodb/index.js

Line 58 in ffe4ace

 next() 

I don't know what this implies.

[Matteo Collina]

PR?

[Tommaso Allevi]

@mcollina done