Do not abort when an ia64 breakpoint slot already holds a break.

ia64_memory_insert_breakpoint() aborts the whole GDB session with an
internal error when the target slot already contains IA64_BREAKPOINT.
This triggers on perfectly ordinary sessions against gdbserver, where a
location can be handed to the insert path twice, and it kills the
session before the user gets any output. SHADOW_CONTENTS is filled from
the breakpoint-hidden read earlier in the function, so it still holds
the original bits and removal restores them; just treat the second
insertion as a no-op.

Signed-off-by: René Rebe <rene@exactco.de>

--- gdb-17.2/gdb/ia64-tdep.c
+++ gdb-17.2/gdb/ia64-tdep.c
@@ -1,6 +1,7 @@
 /* Target-dependent code for the IA-64 for GDB, the GNU debugger.
 
    Copyright (C) 1999-2025 Free Software Foundation, Inc.
+   Copyright (C) 2026 René Rebe <rene@exactco.de>
 
    This file is part of GDB.
 
@@ -716,8 +717,13 @@ ia64_memory_insert_breakpoint (struct gd
      a single instance by update_global_location_list.  */
   instr_breakpoint = slotN_contents (bundle, slotnum);
   if (instr_breakpoint == IA64_BREAKPOINT)
-    internal_error (_("Address %s already contains a breakpoint."),
-		    paddress (gdbarch, bp_tgt->placed_address));
+    {
+      /* The slot already holds a break.  SHADOW_CONTENTS was filled from the
+	 breakpoint-hidden read above, so it still has the original bits and
+	 removal will restore them.  Treat the insertion as a no-op rather
+	 than aborting the whole session.  */
+      return 0;
+    }
   replace_slotN_contents (bundle, IA64_BREAKPOINT, slotnum);
 
   val = target_write_memory (addr + shadow_slotnum, bundle + shadow_slotnum,