Say you allow both Flanking and the Echo Knight. How do they work together?

[Man_Over_Game]

I needed 10 characters, read the title plx and ty.

[Skayaq]

They don't, the echo is not a creature and can not be used to get flanking advantage.

[Kane0]

Uh, you mean the DMG flanking rules? Cause they... arent great.

Anyways, as DM i would rule that your Echo can provide a flank, and incidentally the trickery cleric's duplicate too.
RAW though, they arent creatures and dont count.

[BerzerkerUnit]

Midori Himeno from PS1 game Evil Zone. Her Super move. Watch @ 12m 48s. If you know, you know.

https://m.youtube.com/watch?v=AcaGC2Nwpjw

[OBoyd]

The Trickery Duplicate grants advantage as long as you are both within 5 feet because that's written into the description. No need to "flank" as defined in the DMG. The Echo cannot flank since it's an Object and not an ally.