Hi
All sheep are tagged with the flock number of birth and can not be moved without a tag,in England if you are sending a lamb to slaughter under the age of 12 months then it only needs 1 tag,if you planning on keeping the lamb over 12 months then you need to double tag and 1 of these tags must be electronic,i have just sold some lambs and 1 have double tagged them,1 eid and 1 normal,covers both routes then,you do not need to re tag them when they come to you unless they have lost 1 of the tags they came with.
Some of the other more experienced sheep people will be along soon so if i'm wrong please let me know as this is what i have been following.
Karen