Fix multi-line log when inserting a resource multiple times (#24376)

# Objective

Fixes #24360

## Solution

Flattened the multi-line `warn!()` message in `resource.rs` using the
`\` character, preventing literal newlines and indentation from being
displayed in log output.

## Testing

Created a small program outside the repo that inserts the same resource
twice, which triggered the warning and confirmed that the log output now
appears on a single line.