Two Problems In My Long And Complicated Select Statement!

Hello..
The first problem:
I have 2 tables in MS SQL database: topicstbl and txtfilestbl. And every one have column topicID.
And in both I have fulltext column I want to search in it; but in the first one I want to search in the fulltext column (topicflds) and also in normal columns (topicname and topicsn)
I need to search in these tables and get the (topicID) column as result by 1 table without repeated results.
I tried this code:
SELECT topicID, KEY_TBL.RANK FROM (SELECT topicID, 255 as RANK FROM topicstbl WHERE topicname like '%search word%' or topicsn like '%search word%') AS KEY_TBL
UNION
SELECT topicID, KEY_TBL.RANK FROM topicstbl AS topicstbl INNER JOIN CONTAINSTABLE(topicstbl, topicflds, 'search NEAR word') AS KEY_TBL ON topicstbl.topicID = KEY_TBL.[KEY]
UNION
SELECT topicID, KEY_TBL.RANK FROM txtfilestbl AS txtfilestbl INNER JOIN CONTAINSTABLE(txtfilestbl, txtfile, 'search NEAR word') AS KEY_TBL ON txtfilestbl.txtfileID = KEY_TBL.[KEY] ORDER BY KEY_TBL.RANK DESC

But the problem is the result table has repeated records (same topicID, different Rank Value).
Also I used "SELECT DISTINCT" but same problem.
The second problem:
I want to merge the previous code with ROW_NUMBER code to get results between x and y.
Like this code:
WITH OrderedOrders AS (SELECT *, ROW_NUMBER()
OVER (order by topicID DESC) as RowNumber
FROM topicstbl)

SELECT * FROM OrderedOrders
WHERE RowNumber between 1 and 50
But without change the first order.

Now how to do all these??!