According to the error message, the configuration isn’t correct, duplicate keys are not allowed: theme.
Could you please check the configuration? I guess you’ve executed the echo "theme = 'ananke'" >> config.toml
command twice on the same site.
$ cat config.toml | grep theme
theme = 'ananke'
theme = 'ananke'
If so, remove one of them and try again (hugo server
).