In one aspect, an example method includes receiving, from a first content-presentation device, a request for supplemental content for use in connection with performing a content-modification operation; identifying a download conflict between the first content-presentation device and a second content-presentation device having a same IP address as the first content-presentation device; and providing, to the first content-presentation device, a response to the request, with the request including a download delay instruction. Reception of the download delay instruction by the first content-presentation device causes the first content-presentation device to wait until a condition associated with the download delay instruction is satisfied before downloading a supplemental content item specified in the response.