DBCC REPAIR_REBUILD Fails - Help Required.

I have a customer who has reported some database corruption. The output of
DBCC CHECKDB ('CheckDB') is as follows:

Server: Msg 8935, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. The previous link (1:76577)
on page (1:76578) does not match the previous page (1:253212) that the parent
(1:64277), slot 165 expects for this page.
Server: Msg 8936, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. B-tree chain linkage
mismatch. (1:253212)->next = (1:76578), but (1:76578)->Prev = (1:76577).
Server: Msg 8934, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. The high key value on page
(1:253212) (level 0) is not less than the low key value in the parent (0:1),
slot 0 of the next page (1:76578).
Server: Msg 8978, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. Page (1:253212) is missing a
reference from previous page (1:76577). Possible chain linkage problem.
CHECKDB found 0 allocation errors and 4 consistency errors in table
'Student' (object ID 1685581043).
CHECKDB found 0 allocation errors and 4 consistency errors in database
'CheckDB'.
repair_rebuild is the minimum repair level for the errors found by DBCC
CHECKDB (LincsDB ).

(1 row(s) affected)

Attempting a DBCC CHECKDB ('CheckDB', REPAIR_REBUILD) yields the following
results:

Server: Msg 1505, Level 16, State 1, Line 1
CREATE UNIQUE INDEX terminated because a duplicate key was found for index
ID 2. Most significant primary key is '312037'.
Server: Msg 8935, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. The previous link (1:76577)
on page (1:76578) does not match the previous page (1:253212) that the parent
(1:64277), slot 165 expects for this page.
Server: Msg 8936, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. B-tree chain linkage
mismatch. (1:253212)->next = (1:76578), but (1:76578)->Prev = (1:76577).
Server: Msg 8934, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. The high key value on page
(1:253212) (level 0) is not less than the low key value in the parent (0:1),
slot 0 of the next page (1:76578).
Server: Msg 8978, Level 16, State 1, Line 1
Table error: Object ID 1685581043, index ID 1. Page (1:253212) is missing a
reference from previous page (1:76577). Possible chain linkage problem.
Could not repair this error.
Repairing this error requires other errors to be corrected first.
Repairing this error requires other errors to be corrected first.
Repairing this error requires other errors to be corrected first.
CHECKDB found 0 allocation errors and 4 consistency errors in table
'Student' (object ID 1685581043).
CHECKDB found 0 allocation errors and 4 consistency errors in database
'LincsDB'.
repair_rebuild is the minimum repair level for the errors found by DBCC
CHECKDB (LincsDB repair_rebuild).
The statement has been terminated.

(1 row(s) affected)

I can resolve the issue by dropping and recreating index id: 2 but this
involves dropping and re-creating a lot or related constraints (this is a
primary key).

Can anyone give me any pointers as to why this corruption may have occurred
or more information about the kind of corruption? I have not seen this on any
other customer databases so I don't think it is an application error? Also,
any suggestions on a better (simpler) way to correct this problem?

Unfortunatley, the customers last viable backup is a couple of weeks out of
date and they are reluctant to restore and re-enter all the new data again.

Any help greatly appreciated,
Dave.