A table may be clustered on only one index at a time. This table is already clustered on the index whose name is shown. Before you can cluster on another index, you must execute ALTER INDEX index-name TO NOT CLUSTER. To see which tables are clustered on which indexes, query sysindexes and systables as follows:
SELECT tabname, idxname FROM systables T, sysindexes X WHERE T.tabid = X.tabid AND X.clustered = 'C'