Could be. But there you can create an extra signing key to keep the private key which encrypted the backup very safe.
The problem is described here: "duplicity incr - private key missing".
In short, if you have just the public key on the backup machine but not the private key, a second incremental backup will fail with error message like "The matching private key is missing". This is in fact a bug in Duplicity, which occurs when the system is not configured to use English language - apparently Duplicity checks for certain error messages from GPG and cannot handle translated error messages.
So a possible workaround is mentioned as well: setting environment variable
en_US.UTF8. In fact setting it to an empty value works as well. So starting duplicity like this solved the problem for me under Ubuntu 16.04:
env LANG= duplicity ...