basic principles -- need a citation

I need to cite an authority for a principle: namely, that the degree of
popularity of a name must be taken into account before proposing an
identification between two people with the same name.

Yes, of course, it's obvious and intuitive that if a name is popular,
you shouldn't jump to conclusions just because two contemporaries have
the same name -- unless you have established that the name is rare.
Nonetheless, it is not that uncommon for medievalists to skip the step
of establishing how popular the name is, and jump straight to the
identification on circumstantial evidence. I'm looking for a statement
by an authority or a standard textbook which affirms the necessity for
testing the popularity first -- even though this is to state the
obvious.

Does anyone know of a place where this obvious step is stated in print?

Peter Stewart...

I don't believe there is such a principle to state - if a name is rare,
you still shouldn't jump to conclusions just because two contemporaries
have the same name.

The frequency vs uncommonness of any particular name in surviving
documentary sources may be a matter of record in some cases in a few
places, but unless there is only a single occurrence (with no scope for
confusion) that can't tell us whether or not there could have been two
or more individuals with the same unusual name.

In other words, your idea is basically a matter of practicality and not
principle. "Unusual is not the same as unique" may be a principle,
whereas "rare in statistical relation to common" is just a matter of
degree, and observation if the evidence allows.

Jumping to conclusions is a mistake, or at least a hazardous
enterprise, no matter what the context. If the evidence is sufficent to
the conclusion, no jumping ought to be required.

Peter Stewart