According to this answer there can be issues when there are two tags which are singular and plural form of the same term. However it seems we have both trajectory and trajectories. It looks like the 4 questions with the plural form could just be tagged with the singular and the plurals deleted....

The question Time taken for moon landing has good, thorough answers that can't be posted because the question is currently closed. This is because @JanDoggen voted to close as duplicate and the OP (new user, first question) accepted this judgement. Both of these are unfortunate: Those answers ...