I may have found the answer to my own question;
Filing the T4A slip information (in this case Box 42) seems to automatically fill in the info for the Relevé 1 (RL-1 slip) for the earlier section "T4 / Relevé 1 Employment Income" (in this case Box O code RU).
Therefore, filing the RL-1 slip in the later section "T4A / Relevé 1 / Relevé 2 Pension, Retirement, Annuity" added the same amount again and doubled the "Other income" for the provincial Income Summary.
Maybe someone from TurboTax can confirm.