Check gparts for the same locations as well as parts.

We allow gparts to share locations as that will work, unlike parts (but these may as well be merged, unless they are stars)

Also reduce the verbosity of the reports

src/engine.c

@@ -3360,11 +3360,9 @@ void engine_init_particles(struct engine *e, int flag_entropy_ICs,
   /* Recover the (integer) end of the next time-step */
   engine_collect_timestep_and_rebuild(e, 1);
 
-  /* Check if any particles have the same position and see if we need to
-   * correct the top-level cell h_max to match possible particle updates in
-   * the ghost tasks. Note this must be followed by a rebuild as sub-cells
-   * will not be updated until that is done. */
-  if (s->cells_top != NULL && s->nr_parts > 0) {
+  /* Check if any particles have the same position. This is not
+   * allowed (/0) so we abort.*/
+  if (s->nr_parts > 0) {
 
     /* Sorting should put the same positions next to each other... */
     int failed = 0;
@@ -3373,15 +3371,44 @@ void engine_init_particles(struct engine *e, int flag_entropy_ICs,
       if (prev_x[0] == s->parts[k].x[0] &&
           prev_x[1] == s->parts[k].x[1] &&
           prev_x[2] == s->parts[k].x[2]) {
-        message("Two particles occupy location: %f %f %f",
-              prev_x[0], prev_x[1], prev_x[2]);
-        failed = 1;
+        if (e->verbose)
+          message("Two particles occupy location: %f %f %f",
+                  prev_x[0], prev_x[1], prev_x[2]);
+        failed++;
       }
       prev_x = s->parts[k].x;
     }
-    if (failed)
-      error("Cannot have particles with the same locations");
+    if (failed > 0)
+      error("Have %d particle pairs with the same locations.\n"
+            "Cannot continue", failed);
+  }
+
+  /* Also check any gparts. This is not supposed to be fatal so only warn. */
+  if (s->nr_gparts > 0) {
+    int failed = 0;
+    double *prev_x = s->gparts[0].x;
+    for (size_t k = 1; k < s->nr_gparts; k++) {
+      if (prev_x[0] == s->gparts[k].x[0] &&
+          prev_x[1] == s->gparts[k].x[1] &&
+          prev_x[2] == s->gparts[k].x[2]) {
+        if (e->verbose)
+          message("Two gparts occupy location: %f %f %f / %f %f %f",
+                  prev_x[0], prev_x[1], prev_x[2],
+                  s->gparts[k].x[0], s->gparts[k].x[1], s->gparts[k].x[2] );
+        failed++;
+      }
+      prev_x = s->gparts[k].x;
+    }
+    if (failed > 0)
+      message("WARNING: found %d gpart pairs at the same location. "
+              "That is not optimal", failed);
+  }
 
+  /* Check the top-level cell h_max matches the particles as these can be
+   * updated in the the ghost tasks (only a problem if the ICs estimates for h
+   * are too small). Note this must be followed by a rebuild as sub-cells will
+   * not be updated until that is done. */
+  if (s->cells_top != NULL && s->nr_parts > 0) {
     for (int i = 0; i < s->nr_cells; i++) {
       struct cell *c = &s->cells_top[i];
       if (c->nodeID == engine_rank) {
@@ -3393,6 +3420,7 @@ void engine_init_particles(struct engine *e, int flag_entropy_ICs,
       }
     }
   }
+
   clocks_gettime(&time2);
 
 #ifdef SWIFT_DEBUG_CHECKS