unique constraint on a set With reference to this SO question, what would be a correct way to implement Unique tag Combination Constraint in SQL Server 2005?

To illustrate the problem, I make an example:

A tag_bundle consists of one or more than one tags.
  A unique tag combination can map to a unique tag_bundle, vice versa.

 tag_bundle                   tag            tag_bundle_relation
 +---------------+        +--------+      +---------------+--------+
 | tag_bundle_id |        | tag_id |      | tag_bundle_id | tag_id |
 +---------------+        +--------+      +---------------+--------+
 |       1       |        | 100    |      |       1       |  100   |
 +---------------+        +--------+      +---------------+--------+
 |       2       |        | 101    |      |       1       |  101   |
 +---------------+        +--------+      +---------------+--------+ 
                          | 102    |      |       2       |  101   |
                          +--------+      +---------------+--------+  
                                          |       2       |  102   |
                                          +---------------+--------+


There can't be another tag_bundle having exactly the same combination from tag 100 and tag 101.
There can't be another tag_bundle having exactly the same combination from tag 101 and tag 102.

How can I ensure such unique constraint when executing SQL "concurrently"!! 
that is, to prevent concurrently adding two bundles with exactly the same tag combination 

Adding a simple unique constraint on any table does not work,
Is there any solution other than Trigger or explicit lock.

I come to only this simple way: make tag combination into string, and let it be a unique column.

tag_bundle  (unique on tags)         tag            tag_bundle_relation
 +---------------+-----------+      +--------+      +---------------+--------+
 | tag_bundle_id |  tags     |      | tag_id |      | tag_bundle_id | tag_id |
 +---------------+-----------+      +--------+      +---------------+--------+
 |       1       | "100,101" |      | 101    |      |       1       |  101   |
 +---------------+-----------+      +--------+      +---------------+--------+
                                    | 100    |      |       1       |  100   |
                                    +--------+      +---------------+--------+ 


but it seems not a good way :(

Is there no other solution but with triggers?
My only suggestion is to create stored procedure that performs DML on tag_bundle_relation and restrict all users from manipulating the table directly (it's in a sense 'explicit lock' as you say).  I don't see any other acceptable ways to enforce the required restriction. In my understanding your simple solution just adds complexity to the system - you will need more triggers to synchronize tags field with tag_bundle_relation and tag tables. Also, you may get false fail if you are adding records to tag_bundle_relation one by one - for instance, inserting tag_bundle_id=3 with tag_ids 100,101,102 will fail after attempting to insert 101 (assuming 100 already inserted).