8.2h Contraction preferences.

The contractions for bb, cc, dd, ff, gg, and ea are the least favored of all one-cell contractions because syllable division usually occurs between double consonants. By using other contractions, proper pronunciation is more likely to be preserved. When a choice must be made between contractions, any other one-cell contraction is preferred over them. Examples:

w(ed)d(ing) [not] we(dd)(ing) (of)fice [not] o(ff)ice
m(ed)dle [not] me(dd)le ef(for)t [not] e(ff)ort
sac(ch)(ar)(in)e [not] sa(cc)h(ar)(in)e (the)at(er) [not] (th)(ea)t(er)
le(ar)n [not] l(ea)rn bub(ble) [not] bu(bb)le
b(ea)r [not] b(ea)r me(and)(er) [not] m(ea)nd(er)