How to avoid calling multiple times Wire.begin() in libraries?
What you are interested in is this bit of the Wire library:

TWCR = _BV(TWEN) | _BV(TWIE) | _BV(TWEA);


The important bit is the TWEN bit - TwoWire ENable.

If that bit is set in the TWCR register than Wire.begin() has been called already and you don't need to call it again:

if (TWCR &amp; _BV(TWEN) == 0) {
    Wire.begin();
}


However, it really doesn't matter if you call it multiple times.