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If the existing and <mission> trade orders are for the same quantity of ware, it becomes trickier. In such cases, it's probably best to try to fill both orders to be sure. This is another good reason for having a network of Trade Agents so you can check for existing offers for the ware before accepting a mission (you may gain rep but lose cash by accepting & completing a bad deal). At least the trade offer missions don't currently expire (they're not time-limited), but you'll likely lose profits by not fulfilling their /complete/ requirements ASAP.
Station Defence missions
S+M ships tend to arrive via highways or super highways, whilst L and XL targets boost in. All hostiles are hell-bent on attacking the station so best arrive ASAP or you risk missing the action.
Assassination missions
Typically harder than station defence missions since targets tend to be located in positions in zones that are relatively far from stations. Makes boarding a more realistic option though.