I'm having issues with the SanDisk microSD card I got for my GoPro, they replaced the first one after 3 months (warranty). Now 5 months later the replacement card seems to be failing.

Is this common?

No, that is only common with no-name brands. Does the microSD card support write speeds that equal or exceed the video bitrate on your GoPro? Does the GoPro produce a lot of heat? Have you tried the microSD card in other devices to determine whether the fault is the card or the GoPro?

Common across the format? No.

But not all microSD media is created equal. There were certain revisions of SanDisk mSD cards that were known to fail after they were in retail for a while.

GoPro are write speed sensitive and you need to buy/use the fastest cards. Users have reported SanDisk problems, so many went with other brands like Lexar 633, Samsung Pro, etc. If you want to stay with SanDisk, then you have to look at their Extreme line, class u3 rated that brings them closer to the competition. The alternatives is to either wait for GoPro to release a revision to solve the SanDisk problem like they have done in the past...but if SanDisk comes out with a new, improved card, the problems tend to return. Or use a lower resolution setting so your card can keep pace at whatever write speed it can deliver without errors or loss of data/go into write protected mode.

Update: The user may have been the biggest contributing factor. I was a little pissed and tired trying to get the GoPro ready for a race on Saturday morning (and just got home from a long week of business travel). I blamed the SD card, it appears it was the GoPro. I did a GoPro firmware update and the issue resolved.

Everything worked as expected on Saturday, here is an edit from my race:

The GoPro battery only lasts about an hour when temps are below 0F, otherwise, there'd be more video. This was a 45 mile race, four laps of this single-track course.