Duplicate field values for language fallback content when using language undefined option

On multilingual websites, using the Index language fallback fulltext fields using "language undefined" option will cause fulltext fields to index the same value multiple times.

Steps to reproduce:

1. Create an index and enable the Index language fallback fulltext fields using "language undefined" option
2. Index content title as fulltext
3. Index Language (with fallback) field
4. Enable languages: English, French, Spanish, Russian, Arabic, Chinese (Traditional)
5. Create a page in English, translate it into French
6. Notice that Solr has indexed 2 documents, one for English and one for French

The problem:

The language_undefined title (tm_X3b_und_title) field contains the same value 4 times, once for each untranslated language.

Proposed solution:

When using the Index language fallback fulltext fields using "language undefined" option, only keep the first value in the UND fulltext fields.