There are mods that let you create custom missions with Double XP mutators. There are also mods that let you filter out Double XP missions and have them ready to go almost instantly (see Missions HUD mod). Either of these cases is plausible, but neither of them are game-breaking enough to be punishable. Double XP missions being chained is not very likely, but it happens.
For the record, DRG doesn't have any form of anti-cheat, and what goes on in modded lobbies is not moderated at all, or by anyone. Third-party mod integration allows modded lobbies to show up on the regular server list, but they're almost always tagged as [MODDED] in the lobby name. Make sure you're reading lobby names before you join -- they can be more important than you think.
If you feel like your XP has been "cheated," you can always revert your save. Save files should be stores in your in-game save menu for a week or so.