bugfix: duplicate inputs

diff --git a/shared/psbt.py b/shared/psbt.py
index 0a3da63..31bc40d 100644
--- a/shared/psbt.py
+++ b/shared/psbt.py
@@ -5,6 +5,7 @@
 import stash, gc, history, sys, ngu, ckcc, chains
 from ustruct import unpack_from, unpack, pack
 from ubinascii import hexlify as b2a_hex
+from ucollections import OrderedDict
 from utils import xfp2str, B2A, keypath_to_str
 from utils import seconds2human_readable, datetime_from_timestamp, datetime_to_str
 from chains import NLOCK_IS_TIME
@@ -1761,7 +1762,21 @@ class psbtObject(psbtProxy):
         foreign = []
         total_in = 0
 
+        prevouts_max_len = 50
+        prevouts = OrderedDict()
+
         for i, txi in self.input_iter():
+            # check for duplicate inputs
+            if len(prevouts) >= prevouts_max_len:
+                first = next(iter(prevouts))  # O(1) in mpy
+                del prevouts[first]
+
+            k = txi.prevout.n, txi.prevout.hash
+            if k in prevouts:  # O(1)
+                raise FatalPSBTIssue("Duplicate inputs!")
+
+            prevouts[k] = True
+
             inp = self.inputs[i]
             if inp.fully_signed:
                 self.presigned_inputs.add(i)
diff --git a/testing/test_sign.py b/testing/test_sign.py
index 06f10e1..d98540b 100644
--- a/testing/test_sign.py
+++ b/testing/test_sign.py
@@ -3646,4 +3646,17 @@ def test_txn_nVersion_zero(segwit, fake_txn, start_sign, cap_story, goto_home):
     assert title == "Failure"
     assert "txn version" in story
 
+@pytest.mark.parametrize("segwit_in", [True, False])
+@pytest.mark.parametrize("num_ins", [2, 50, 51])
+def test_duplicate_inputs(segwit_in, num_ins, fake_txn, start_sign, end_sign, cap_story):
+    psbt = fake_txn(num_ins, 2, segwit_in=segwit_in, dupe_ins=[num_ins-1])
+    start_sign(psbt)
+    title, story = cap_story()
+    if num_ins <= 50:
+        # only works if duplicate is no longer than 50 inputs from original
+        assert "Duplicate inputs!" in story
+    else:
+        # does not work
+        assert title == "OK TO SEND?"
+
 # EOF
diff --git a/testing/txn.py b/testing/txn.py
index 3f7937d..17a7551 100644
--- a/testing/txn.py
+++ b/testing/txn.py
@@ -25,7 +25,11 @@ def fake_txn(dev, pytestconfig):
              outstyles=['p2pkh'],  psbt_hacker=None, change_outputs=[],
              capture_scripts=None, add_xpub=None, op_return=None, taproot_in=False,
              psbt_v2=None, input_amount=1E8, unknown_out_script=None, lock_time=0,
-             sequences=None, sighashes=None):
+             sequences=None, sighashes=None, dupe_ins=[]):
+
+        # dupe_ins cannot contain zero, as that will be the duplicated input
+        if dupe_ins:
+            assert 0 not in dupe_ins
 
         psbt = BasicPSBT()
 
@@ -58,12 +62,17 @@ def fake_txn(dev, pytestconfig):
             # - each input is 1BTC
 
             # addr where the fake money will be stored.
-            subkey = mk.subkey_for_path(subpath % i)
+            if i in dupe_ins:
+                # always duplicate zeroth input
+                subkey = mk.subkey_for_path(subpath % 0)
+            else:
+                subkey = mk.subkey_for_path(subpath % i)
             sec = subkey.sec()
             assert len(sec) == 33, "expect compressed"
             assert subpath[0:2] == '0/'
 
-            psbt.inputs[i].bip32_paths[sec] = xfp + struct.pack('<II', 0, i)
+            idx = 0 if i in dupe_ins else i
+            psbt.inputs[i].bip32_paths[sec] = xfp + struct.pack('<II', 0, idx)
 
             # UTXO that provides the funding for to-be-signed txn
             supply = CTransaction()
@@ -93,7 +102,7 @@ def fake_txn(dev, pytestconfig):
 
             if segwit_in:
                 # just utxo for segwit
-                psbt.inputs[i].witness_utxo = supply.vout[-1].serialize()
+                psbt.inputs[i].witness_utxo = supply.vout[0 if i in dupe_ins else -1].serialize()
             else:
                 # whole tx for pre-segwit
                 psbt.inputs[i].utxo = supply.serialize_with_witness()