diff --git a/src/ballet/txn/fd_txn_parse.c b/src/ballet/txn/fd_txn_parse.c
index 9652c99e104..2e3bed3d277 100644
--- a/src/ballet/txn/fd_txn_parse.c
+++ b/src/ballet/txn/fd_txn_parse.c
@@ -137,6 +137,14 @@ fd_txn_parse_core( uchar const             * payload,
 
     CHECK_LEFT( FD_TXN_ACCT_ADDR_SZ*acct_addr_cnt ); ulong acct_addr_off  =          i  ;     i+=FD_TXN_ACCT_ADDR_SZ*acct_addr_cnt;
 
+    /* Throw out transactions with duplicate addresses at parse time.
+       Agave rejects these at sanitisation time. */
+    for( ulong j=0UL; j<(ulong)acct_addr_cnt; j++ ) {
+      for( ulong k=j+1UL; k<(ulong)acct_addr_cnt; k++ ) {
+        CHECK( memcmp( payload+acct_addr_off+FD_TXN_ACCT_ADDR_SZ*j, payload+acct_addr_off+FD_TXN_ACCT_ADDR_SZ*k, FD_TXN_ACCT_ADDR_SZ ) );
+      }
+    }
+
     /* Config values: 4 bytes per set mask bit */
     ulong config_values_off = i;
     ulong num_config_values = (ulong)fd_uint_popcnt( config_mask );
diff --git a/src/ballet/txn/test_txn_parse.c b/src/ballet/txn/test_txn_parse.c
index 47c879877fa..18682f99d1b 100644
--- a/src/ballet/txn/test_txn_parse.c
+++ b/src/ballet/txn/test_txn_parse.c
@@ -43,7 +43,7 @@ build_legacy_one_instr( uchar * buf, ushort instr_acct_cnt ) {
   buf[ o++ ] = 0x01;                                         /* header: num_readonly_unsigned = 1    */
   buf[ o++ ] = 0x02;                                         /* account address count = 2            */
   for( ulong a=0UL; a<2UL; a++ )
-    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)(a*32UL+j); /* 2 addresses                   */
+    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)( j ? 0xA0UL+j : a ); /* 2 distinct addresses  */
   for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)(0xB0+j);     /* recent blockhash              */
   buf[ o++ ] = 0x01;                                         /* instruction count = 1                */
   buf[ o++ ] = 0x01;                                         /* program_id index = 1                 */
@@ -307,7 +307,7 @@ build_v1( uchar       * buf,
   buf[ o++ ] = num_addr;
   /* addresses */
   for( ulong a=0UL; a<num_addr; a++ )
-    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)(a*32UL+j);
+    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)( j ? 0xA0UL+j : a ); /* byte 0 = index so addresses are distinct */
   /* config values: one 4-byte word per set bit.  When the heap-size bit
      (bit 4, the highest bit) is set its word is last and must hold a valid
      heap size, so use 64 KiB; the other words are arbitrary. */
@@ -463,6 +463,33 @@ void txn_v1_correctness( void ) {
     FD_TEST( 0UL == fd_txn_parse( v1_buf, sz, out_buf, &counters ) );
   }
 
+  /* duplicate address (address 2 is a copy of address 1) */
+  {
+    ulong sz = build_v1( v1_buf, 1, 0, 1, 0u, 3, ix1, 1 );
+    ulong addr_off = 1UL+3UL+4UL+32UL+1UL+1UL;
+    fd_memcpy( v1_buf+addr_off+2UL*32UL, v1_buf+addr_off+1UL*32UL, 32UL );
+    counters = (fd_txn_parse_counters_t){0};
+    FD_TEST( 0UL == fd_txn_parse( v1_buf, sz, out_buf, &counters ) );
+  }
+
+  /* duplicate address at the far end (address 63 is a copy of address 0) */
+  {
+    ulong sz = build_v1( v1_buf, 1, 0, 1, 0u, 64, ix1, 1 );
+    ulong addr_off = 1UL+3UL+4UL+32UL+1UL+1UL;
+    fd_memcpy( v1_buf+addr_off+63UL*32UL, v1_buf+addr_off, 32UL );
+    counters = (fd_txn_parse_counters_t){0};
+    FD_TEST( 0UL == fd_txn_parse( v1_buf, sz, out_buf, &counters ) );
+  }
+
+  /* num_addr == FD_TXN_ACCT_ADDR_MAX (64) distinct addresses is accepted */
+  {
+    ulong sz = build_v1( v1_buf, 1, 0, 1, 0u, 64, ix1, 1 );
+    counters = (fd_txn_parse_counters_t){0};
+    FD_TEST( fd_txn_parse( v1_buf, sz, out_buf, &counters ) );
+    FD_TEST( counters.success_cnt==1UL );
+    FD_TEST( parsed->acct_addr_cnt == 64 );
+  }
+
   /* num_addr exceeds FD_TXN_ACCT_ADDR_MAX (64) */
   {
     ulong sz = build_v1( v1_buf, 1, 0, 1, 0u, 65, ix1, 1 );
diff --git a/src/disco/keyguard/test_keyguard.c b/src/disco/keyguard/test_keyguard.c
index 77cc2af168c..fff5718a8f9 100644
--- a/src/disco/keyguard/test_keyguard.c
+++ b/src/disco/keyguard/test_keyguard.c
@@ -20,7 +20,7 @@ build_txn_v1( uchar * buf,
   buf[ o++ ] = (uchar)instr_cnt;
   buf[ o++ ] = (uchar)num_addr;
   for( ulong a=0UL; a<num_addr; a++ )
-    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)(a*32UL+j);
+    for( ulong j=0UL; j<32UL; j++ ) buf[ o++ ] = (uchar)( j ? 0xA0UL+j : a );
   /* instruction headers: program id 1, no accounts, no data */
   for( ulong x=0UL; x<instr_cnt; x++ ) {
     buf[ o++ ] = (uchar)1;  /* program id */