There *is* a case where the same spelling have different pronunciations, and it’s one that takes Greeks by surprise.

In vernacular Greek, /i/ before vowels is reduced to [j]; so /ia/ ends up a single syllable, [ja].

In learnèd Greek, /i/ before vowels remained a full vowel; so /ia/ ends up as two syllables, [i.a].

There are several instances of words that are spelled identically, and often that are cognate, but that have a distinct learnèd and a vernacular meaning—and distinct pronunciations associated with them.

Three examples:

• λόγια. Vernacular [loɣja] > [loʝa] “words” (plural of ancient logion, “little word”). Learnèd [loɣi.a] “learnèd” (feminine singular and neuter plural of logios “having to do with words, with reason”)
• βιάζω. Vernacular [vjazo] “to hasten”. Learnèd [vi.azo] “to rape”. (Both derived from the original sense “to force”. )
• άδεια. Vernacular [aðja] “empty”. Learnèd [aði.a] “leave, day off” (because it is an “empty” day.)

You will occasionally see writers online attempt to differentiate between the two pronunciations by putting a diaeresis over the iota (λόγια vs λόγϊα), but that use of the diaeresis has no sanction in official Greek orthography, and most writers would regard it as wrong.