[jihtsan]

Fixes #98.

The default header and {{.Import}} can emit the same aliased import in separate blocks. After goimports resolves the generated source, remove duplicate import bindings by decoded path and explicit alias. Keep the first binding, distinct aliases, comments, custom headers and the existing template helper compatible. No template changes are required.

Validation on Go 1.24.2:

• Both duplicate-generation regression cases fail on the original implementation and pass with this change; the distinct-alias control passes.
• go test -count=1 ./..., go test -race ./..., go vet ./... and go build ./... pass.
• The locked golangci-lint v1.64.8 command from the Makefile passes: golangci-lint run --enable=goimports --disable=unused --exclude=S1023,'Error return value' ./....
• Generated the Prometheus wrapper from the issue author's public reproduction with the built CLI. It now contains one dtoAlias import and go test ./... compiles it.
• An independent public-generator harness verifies nine boundary cases, including cross-block imports, distinct aliases, custom headers, raw/quoted paths and comment preservation.

Validation ran on macOS; Linux and Windows were not run locally.

[hexdigest]

Thank you @jihtsan !

This one has been bothering me for a very long time.