diff --git a/src/kyupy/sim.py b/src/kyupy/sim.py
index 29ac420..1602989 100644
--- a/src/kyupy/sim.py
+++ b/src/kyupy/sim.py
@@ -279,7 +279,7 @@ class SimOps:
 
         # allocate memory for the rest of the circuit
         for op_start, op_stop in zip(self.level_starts, self.level_stops):
-            free_list = []
+            free_set = set()
             for op in self.ops[op_start:op_stop]:
                 # if we fork-strip, always take the stems
                 i0_idx = stems[op[2]] if stems[op[2]] >= 0 else op[2]
@@ -290,15 +290,15 @@ class SimOps:
                 ref_count[i1_idx] -= 1
                 ref_count[i2_idx] -= 1
                 ref_count[i3_idx] -= 1
-                if ref_count[i0_idx] <= 0: free_list.append(self.c_locs[i0_idx])
-                if ref_count[i1_idx] <= 0: free_list.append(self.c_locs[i1_idx])
-                if ref_count[i2_idx] <= 0: free_list.append(self.c_locs[i2_idx])
-                if ref_count[i3_idx] <= 0: free_list.append(self.c_locs[i3_idx])
+                if ref_count[i0_idx] <= 0: free_set.add(self.c_locs[i0_idx])
+                if ref_count[i1_idx] <= 0: free_set.add(self.c_locs[i1_idx])
+                if ref_count[i2_idx] <= 0: free_set.add(self.c_locs[i2_idx])
+                if ref_count[i3_idx] <= 0: free_set.add(self.c_locs[i3_idx])
                 o_idx = op[1]
                 cap = max(c_caps_min, c_caps[o_idx])
                 self.c_locs[o_idx], self.c_caps[o_idx] = h.alloc(cap), cap
             if c_reuse:
-                for loc in free_list:
+                for loc in free_set:
                     h.free(loc)
 
         # copy memory location and capacity from stems to fanout lines