PSA - If you haven't heard yet, putting mid as your secondary...

Pretty much guarantees you get your primary role. If you want to duo bot, have whoever is playing adc select mid as secondary, while the support picks adc as his secondary. You will get your duo bot 90%+ of the time.

Mid>Top>Adc>Jungle>Support>Fill === Order of popularity. Choosing something more popular than your primary as your secondary pretty much guarantees you the role you want. Mid laners have the least likely chance of receiving their role.

Ex: If you want to play a duo with mid and jungle, your best bet is to have the midlaner choose top as his secondary, while the jungler chooses mid.

If Riot is going to insist to push these changes regardless of whether or not they were wanted, than they shouldn't be surprised when players find ways around it.

EDIT: As others point out there is an element of luck. You are one of thousands in queue. This is just a way to put the odds greatly in your favor.

EDIT 2: NO. Having a ADC/SUP SUP/ADC combo doesn't work. Your duo has to have a minimum of three different roles or you can't leave the lobby.