Anybody with the name D. has the ability to consume more than one Devil fruit. That's what BB figured out (presumably) before any of the other D's. The D stands for devil and if you're a descendant of the D then you have the ability to have more than one Akuma no Mi.
Luffy, after the events of Fishman Island, will visit his brother Ace's grave where he will see the Mera Mera no Mi growing out of the ground. At first Luffy will take the fruit for sentimental value, but through one way or another he'll become aware of the D's potentially being able to have more than one devil fruit. Understanding this, he'll eat the Mera Mera no Mi ( "Ace won't let anything happen to me" ) and will now have both the abilities of the Rubberman and the Fireman, but not all of them. Much like BB can only use his Gura Gura no Mi in one hand, the Mera will now have similar restrictions. Luffy will not be able to transform his whole body into fire like all of the logias we've seen so far because his body is still made entirely out of rubber seeing as that was the first fruit he cosumed, instead he'll now have the ability to project flame blasts out of his hands and also this will give him the ability to create a much stronger and durable Gear 4th. Luffy will focus the flames into a certain area thus vulcanizing his rubber body causing it to become extremely tough and hard, making his punches a lot stronger/harder. He'll use the Mera Mera no Mi primarily in ways that would enhance the abilities of the Gomu Gomu no Mi, but not always.