So how come i never see duo mid??

Double mid makes perfect sense

It's the lane that gets XP & Gold the fastest and is closest to all objectives and has easy access to roaming

Traditionally mages get send there for a level lead since they gain more power from that then items, well at least a lot faster.

However having your adc + support mid makes sense too since it's easy for the support to roam from there and ward the map as well as team up with the jungler to gank other lanes or take down neutrals, and ofc place deep wards.

Also with 2v1 mid you can keep the enemy mage down in gold.

There are other possibilities too, you do not need to send you adc with the support mid.

You could send 2 supports mid instead and send the adc top, it's isolated but does grant gold and xp since the adc is not very relevant early on this is a decent spot too, ADC's are pretty good laners in general they just lack overall power to really do anything relevant so early. though they are by no means weak. This does however require a mobile adc for safety.

For the mid lane you would simply pick a support combo with sustain and wave clear who can suppress the enemy mage/assassin and light up the map with roams, bard would be a good pick since he can roam bot and drop off some shrines there to help him out, assuming he has to go 1v2.

Having 2 supports mid gives you the option of selecting the biggest early game lane bully adc's for the to plane since you have 2 supports to beef them up to god status in the late game. this allows you to shut down the enemy top laner in some situations :S

The 2 mids can roam around to help either side lane sending 1 guy over as needed, or going both if necessary.

You simply send your tank into the jungle, he supports the side lanes in between farm sessions. you will always have at least 3 people on either side of the map to disrupt enemy objective taking too, and their mid can't help without at least 1 guy following him around while the other takes his tower.

The bot lane is where your mage will go, although you are not required to pick one ofc, as long as you have relatively safe wave clear and some cc you should be good to go. since this is the lane where you get your xp advantage.

Once the enemy has pushed in your jungler and support(s) can come gank their asses sharing gold and xp is not an issue for the double support lane either. meanwhile all other roles get full XP and gold. The XP advantage in this case goes to your adc.

This double support mid is more about taking away agency from all the earl & /mid game positions of the enemy by keeping those down.

with the jungler and double mids in position to help either lane or thwart neutrals it becomes really hard for the enemy to gank/camp or go for those. Additionally you get to keep down their mid and top laner in gold. Their one ''winning lane'' will still be faced by the impending ganks from jungle and mid lane roams because they will always be pushing in while the mage down there gains an xp advantage over his opponents to keep him relevant damage wise when those ganks happen. this forces the enemy duo lane to play safe which in turn grants some safety for your solo bot lane. Even if the enemy jungle comes down there you can keep mid covered while sending over 2 people to help defend. Counter jungling the tank jungler is useless as he will be relevant anyway and he always has a support nearby to back him up.

You can always send someone over to the botlane to help out for a minute or cover during a back

Somehow these kind of strategies always seemed to make sense to me but i never see it happen.