A thread-specific breakpoint really only needs to have locations for the address/program space of the inferior which the thread belongs to.  However, we currently expand the breakpoint spec in every program space anyhow.  Vis:

 (gdb) b main thread 1.1
 Breakpoint 1 at 0x1285: main. (2 locations)
 (gdb) info breakpoints 
 Num     Type           Disp Enb Address            What
 1       breakpoint     keep y   <MULTIPLE>          thread 1
         stop only in thread 1.1
 1.1                         y   0x0000000000001285 in main at 
 gdb.multi/goodbye.c:61 inf 2
 1.2                         y   0x00005555555551f0 in main at 
 gdb.multi/hello.c:51 inf 1

Location 1.1 above is worse than useless, as it'll end up inserted and cause inferior 2 to unnecessarily stop only to always be re-resumed.

We should instead end up with only one location for inferior 1, just like if inferior 2 did not exist, like:

 (gdb) b main thread 1.1
 Breakpoint 1 at 0x5555555551f0: file  gdb.multi/hello.c, line 51.
 (gdb) info breakpoints 
 Num     Type           Disp Enb Address            What
 1       breakpoint     keep y   0x00005555555551f0 in main at 
 gdb.multi/hello.c:51 thread 1
         stop only in thread 1