A core dump of a multithreaded process always includes the

PID in the core filename.

man5/core.5

@@ -127,6 +127,16 @@ The
 .BR gdb (1)
 .I gcore
 command can be used to obtain a core dump of a running process.
+
+If a multithreaded process (or, more precisely, a process that
+shares its memory with another process by being created with the
+.B CLONE_VM
+flag of
+.BR clone (2))
+dumps core, then the process ID is always appended to the core filename, 
+unless the PID was otherwise included as a consequence of a 
+%p specification in
+.IR /proc/sys/kernel/core_pattern .
 .SH SEE ALSO
 .BR gdb (1),
 .BR getrlimit (2),