Additional collects may be prayed, beginning with the COLLECT OF THE DAY (or from the previous Sunday when there is a feast day), followed by other collects, and concluding with the COLLECT FOR PEACE. In choosing other collects, the liturgist will be mindful of what has already been prayed in the Litany. LSB Altar Book, p. 346.
That is, the litany in Evening Prayer has just prayed for almost every conceivable need. Hence, the "may" of this rubric, which permits, but certainly does not require, the addition of other collects prior to praying the Collect for Peace. My suggestion is that if one is "mindful of what has already been prayed in the litany" one will not avail himself of the opportunity to ask again what has already been petitioned; I'd just always close with the Collect for Peace and call it done. If by chance the litany DID forget a specific petition, the Lord's Prayer that follows will certainly cover it!