[QuickStatements] Allows duplicate time values

[Laddo]

For time snacks representing years between -990 to +990, QuickStatements does not recognize that there is already a statement with the same value on a Wikidata item and creates a double.

For example, trying these in QS will actually add three statement copies (precision=decade): Q79796 P585 +0000000930-01-01T00:00:00Z/8 Q79796 P585 +0000000930-01-01T00:00:00Z/8 Q79796 P585 +0000000930-01-01T00:00:00Z/8