This was done on a server

You place a block or chest in the nether, go back through the portal, then go back in the nether and break said block/chest then head back to the overworld. Once you go back to the nether you will find the destroyed block.

Note; This is chunk based, If you try doing the same dupe in the same chunk, the new blocks will be overridden by the old dupe. Therefore go to a different chunk to make a different block dupe.

It apears that the nether must be unloaded for this to occur. There must not be anyone in the nether when attempting this

Was hard to replicate again, I believe it was in relation to a laggy server aswell but nonetheless am unsure of the real cause.

If needed, I can record and upload a video of this happening