What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.149.0-66240338f1b908ca3b163384c8229943e74eb290+extended darwin/arm64 BuildDate=2025-08-27T15:37:16Z VendorInfo=gohugoio

Does this issue reproduce with the latest release?

Yes

Describe the bug

When setting a Hugo configuration value of type list via an environment variable, the parsing result depends on whether the config file already defines the key as a list. If the key does not exist or is undefined, the environment variable is interpreted as a single string. If the key exists as a list, the environment variable is correctly parsed as a list.

Reproduce

1. Clone a test repository (arbitrary example):

git clone https://github.com/jmooring/hugo-testing --single-branch --branch=hugo-forum-topic-55568
cd hugo-testing

2. Without defining ignoreFiles in the config:

HUGO_IGNOREFILES='[foo,bar]' hugo config | grep ignore
HUGO_IGNOREFILES='["foo","bar"]' hugo config | grep ignore

Result:

ignorefiles = ['[foo,bar]']

3. Create an empty list in the config:

(echo 'ignoreFiles = []'; cat hugo.toml) > tmp && mv tmp hugo.toml

4. Run the same command again:

HUGO_IGNOREFILES='[foo,bar]' hugo config | grep ignore
HUGO_IGNOREFILES='["foo","bar"]' hugo config | grep ignore

Result:

ignorefiles = ['foo', 'bar']

Expected behavior

Environment variables defining list-type config keys should be parsed consistently regardless of whether the key exists in the config file.