Block duplicate app directives (`#:`)

[jjonescz]

From the spec:

We disallow duplicate #: directives to allow us design some deduplication mechanism in the future. Specifically, directives are considered duplicate if their type and name (case insensitive) are equal. Later with deduplication, separate "self-contained" utilities could reference overlapping sets of packages even if they end up in the same compilation. For example, properties could be concatenated via ;, more specific package versions could override less specific ones.

See also #48437 (comment).