Currently, using the insert command of this script on a file like
wallet_zapwallettxes.py will caues part of the copyright header to be
duplicated.
i.e. from:
```
# Copyright (c) 2014-2016 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
```
to:
```
# Copyright (c) 2015-2019 The Bitcoin developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
# Copyright (c) 2014-2016 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
```
This change fixes the behavior to exclude the duplicated lines.