[catwhowalks]

So far, Go-Onger is the only clear case of two sixth rangers. There are other, but they are highly questionable at best. I consider cases of 3 rangers with 2 added later(3+2) to be something different, but to add all that I am aware of. There are also Extra Heroes, heroes that have powers and appearances distinct from rangers, but join the team. Some teams have had both a Extra Hero and a sixth ranger.

Liveman--3+2
Carranger--1 Extra hero who more or less joins the team, plus a second one who claims to be the sixth ranger. Isn't really and only shows up 3 times though.
Hurricenger--3+2+ a regular sixth ranger on top of that.
Dekaranger--1 regularly appearing extra ranger(officially categorized as an Extra Hero, but fits as a ranger in all respects) a regular extra ranger, and 3 different 1 shot guest rangers, all female, interestingly enough.
Boukenger--Both a regular Sixth Ranger and an Extra Hero
Gekiranger--3+1+1, the only such case. At the very end of the show, two of the villains switch sides and are consider Extra Heroes.
Magiranger--1 regular sixth ranger, plus both parents who are officially considered Extra Heroes. Techically, the sixth ranger is actually an extra hero, but its a bit murky.
Go-Onger--3+2(although in the very first episodes the 2 are added) plus 2 sixth rangers. Again, the only clear example.
Go-Buster--standard 3+2
Kyuoruger--1 standard sixth ranger, plus 4 recurring rangers for a total of 10. Notable in that the sixth ranger was added after some of the others were introduced. One of the extra ranger, Violet is more or let a main character, but is the tech support and only rarely transforms. The main 6 are also the only ones who don't transfer powers to another character during the show. It's an odd case.
EDIT: Forgot one. Abarangers--an odd case in which one ranger is introduces, immediately loses his powers in the first few minutes, then the others 3 are introduced. The first one gets his powers back later and in the last couple of episodes, the evil ranger switches sides and join them. He's officially the sixth ranger, but Abbareblack arguably fits as well.