Mark 8:17

In Mark 8:17 the words νοειτε and συνιετε forced the Latin and Gothic translators to choose different renderings from the ones they usually choose (νοεω/συνιημι are both translated in all other instances in the NT: Vulgate/Vetus Latina: intellegere /Gothic: frathjan), to avoid the in Mk 8:17 impossible repetition of the standard-translating.

Did the coptic translators faced a comparable problem? (The Horner translation seems to show a difference between North and South, but not knowing any coptic, I cannot judge this).

Yes, both the Sahidic and Bohairic translations use two different verbs in Mk 8:17.

S ⲛⲧⲉⲧⲛⲛⲟⲉⲓ ⲁⲛ ⲟⲩⲇⲉ ⲛⲧⲉⲧⲛϯ ⲁⲛ ⲛϩⲧⲏⲧⲛ (Quecke 1972)
“You don’t understand nor do you pay attention”

B ⲙⲡⲁⲧⲉⲧⲉⲛⲉⲙⲓ ⲟⲩⲟϩ ⲧⲉⲧⲉⲛⲕⲁϯ (Horner)

ⲛⲟⲉⲓ is from Gr. νοεῖν of course, a pretty common verb in Coptic; ϯ-ϩⲧⲏ⸗ “observe, notice, pay attention, heed” (lit. “to give one’s mind”, see CD 716a for more details).