Duplicate values not removed before saving field

Problem/Motivation

If a value entered in the "Other" area duplicates a value already stored in the field, duplicate values get stored in the database. The problem is particularly likely to come up with fields with longer option lists and/or when this module is used in conjunction with Chosen module to create a "free tagging" style interface.

Steps to reproduce

1. Create a "list" field (text or integer) and allow multiple values. Example: "favorite colors"
2. Create an allowed values list. Example: "Red, Green, Blue"
3. Configure field to use Select or Other.
4. Create a test node. Select one or more of the allowed values ("Green") AND select the "Other" option.
5. In the "other" field, enter one of the selected, existing values ("Green").
6. Save and view the node. The value entered in "Other" field ("Green") is now displayed twice.

Proposed resolution

Add code to remove duplicate values. The attached patch uses array_unique and seems to fix the problem for text and integer fields.

Remaining tasks

• Confirm the behavior produced by this change is desired: duplicate entries should not be created.
• Patch needs review and testing.
• Consider if the code that removes duplicates should be case-sensitive or not. It is currently case-sensitive, so in the above example "green" would be allowed as a new entry but "Green" would be considered a duplicate.