Conflicts and overrides

Conflicts between arguments, be them options, positional arguments or commands, can arise for a variety of reasons:

Two options have the same name (either long or short)
Two arguments have the same destination key, but different types (e.g. one is Any and the other String)
Two arguments have the same destination key, but incompatible actions (e.g. one does :store_arg and the other :append_arg)
Two positional arguments have the same metavar (and are therefore indistinguishable in the usage and help screens and in error messages)
An argument's destination key is the same as a command name
Two commands with the same name are given, but one has a long-option form (e.g. test and --test)
A command alias is equal to another command's name or alias

When the general setting error_on_conflict is true, or any time the specific force_override table entry setting is false, any of the above conditions leads to an error.

On the other hand, setting error_on_conflict to false, or force_override to true, will try to force the resolution of most of the conflicts in favor of the newest added entry. The general rules are the following:

In case of duplicate options, all conflicting forms of the older options are removed; if all forms of an option are removed, the option is deleted entirely
In case of duplicate destination key and incompatible types or actions, the older argument is deleted
In case of duplicate positional arguments metavars, the older argument is deleted
A command can override an argument with the same destination key
However, an argument can never override a command; neither can a command override another command when added with @add_arg_table! (compatible commands are merged by import_settings! though)
Conflicting command aliases are removed