i am encountering it in my work too, i see it on a firehose feeder i made that pulls events from multiple relays concurrently and processes the unique ones it sees in a window of 2048 events (uses a map to implement a uniqueness invariant)

it's very easy to detect them, you literally would just look for the e, , reply tag in it, and go to the event named in the event id, and check that the content field has the same prefix data and ignore it after that

but it's quite funny, very often the reply events are showing up ahead of the replied to events in the feed

i'm sure that replyguy is just the tip of the iceberg of problems these free relays are having