Docker tag command append new tag to old one? Keep getting invalid reference format errors

Good day folks,

I’m trying to add a new tag to an existing image but I keep getting the dreaded invalid reference format error. Is this even possible to add a new tag to an existing one?
Here’s an example to illustrate what I’m trying to accomplish, basically an image tag value of docs-image:latest

docker tag server:5075/main:docs-image server:5075/main:docs-image:latest

Thanks for the help and any tips. All the best