MemoryIndex does not allow user to add multiple values for a single field name

When using MemoryIndex.addField the following operation throws an IllegalArgumentException:

index.addField("foobar", "value1", LuceneAnalyzer);
index.addField("foobar", "value2", LuceneAnalyzer);

This throws:
java.lang.IllegalArgumentException: field must not be added more than once

According to Uwe Schindler on the java-user mailing list this violates the IndexWriter/IndexReader contract.