Place cantaloupe and cucumbers in a colander; set a plate on top. Place a heavy object such as a large can of food on top of plate to squeeze out excess liquid from cantaloupe and cucumbers, tossing about every 30 minutes, 2 to 3 hours.
Transfer cantaloupe and cucumbers to a large bowl; add onion, celery, poppy seed dressing, and red wine vinegar. Toss to coat. Stir in poppy seeds.
