diff --git a/day-19/day-19.py b/day-19/day-19.py
index 565c7af..7dd83ba 100644
--- a/day-19/day-19.py
+++ b/day-19/day-19.py
@@ -133,15 +133,15 @@ def solve(input):
         s['beacons'] = b
         s['metrics'] = calc_distances(b)
         s['origin'] = (0, 0, 0)
-        s['overlapping'] = []
+        s['overlapping'] = set()
         scanners.append(s)
     for i in range(len(scanners)):
         for j in range(i + 1, len(scanners)):
             num = num_overlapping(
                 scanners[i]['metrics'], scanners[j]['metrics'])
             if num:
-                scanners[i]['overlapping'].append(j)
-                scanners[j]['overlapping'].append(i)
+                scanners[i]['overlapping'].add(j)
+                scanners[j]['overlapping'].add(i)
     transform(scanners, 0)
     m = set()
     for s in scanners: