Which version of Ignition is this happening in and is it only a specific type of tag?
In 8.3, we have a known issue where memory tags on startup are getting a duplicate history record processed because we are persisting the timestamp when it last changed after a restart now.
I would recommend reaching out to to support regarding the issue you are seeing. In looking at what has been fixed since the release you are currently using, there is 1 bug that could be related:
Fixed in 8.1.27: Fixed issue where duplicate timestamps would be created when OPC quality changes.