Which duplicate setting survives after layers are sorted by precedence.
Layers are resolved in increasing precedence order: BASE, then TUNE, then
USER. A duplicate is either a parsed command with the same normalized key,
or an unparsed command with the same normalized full text.
FIRST keeps the first duplicate encountered. This preserves lower-precedence
defaults when later layers repeat the same setting.
LAST keeps the last duplicate encountered. This is the normal generator
behavior: tune settings override base settings, and user settings override
both.
Definition at line 12 of file GeneratorSettingsSemantics.py.