The document validation function does not check the uniqueness of SPDX identifiers

[nikpivkin]

Hi! According to the SPDX specification, the SPDX identifier must be unique, but the document validation function does not check this. It would be nice to add such a check, since the validation tool on the SPDX website warns about this.

Ref:

• https://spdx.github.io/spdx-spec/v2.3/file-information/#82-file-spdx-identifier-field
• https://spdx.github.io/spdx-spec/v2.3/package-information/#72-package-spdx-identifier-field