Records with identical uid and timestamp in node_revision
DISCLAIMER : Not a Drupal Developer, just a pure MySQL DBA
Here is a sample node_revision table:
mysql&gt; show create table node_revisions\G
*************************** 1. row ***************************
       Table: node_revisions
Create Table: CREATE TABLE `node_revisions` (
  `nid` int(10) unsigned NOT NULL DEFAULT '0',
  `vid` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL DEFAULT '0',
  `title` varchar(255) NOT NULL DEFAULT '',
  `body` longtext NOT NULL,
  `teaser` longtext NOT NULL,
  `log` longtext NOT NULL,
  `timestamp` int(11) NOT NULL DEFAULT '0',
  `format` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`vid`),
  KEY `nid` (`nid`),
  KEY `uid` (`uid`)
) ENGINE=InnoDB AUTO_INCREMENT=227825 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql&gt;

Strictly in terms of MySQL, there is nothing in place to prevent the same UID and timestamp from entering this table multiple times. The index uid is a non-unique index on the uid column. What you need is a UNIQUE INDEX to prevent this at the database level.
Run this query:
SELECT uid,timestamp,COUNT(1) ut_dups FROM node_revisions
GROUP BY uid,timestamp HAVING COUNT(1) &gt; 1;

and this will uncover other UIDs with the same timestamp issue.
If you want to block rows from entering this table with duplicate (uid,timestamp), change the indexes as follows (making a backup first):
#
# Backup node_revision
#
CREATE TABLE node_revision_backup LIKE node_revision;
INSERT INTO node_revision_backup SELECT * FROM node_revision;
#
# Change Indexes
#
CREATE TABLE node_revision_new LIKE node_revision;
ALTER TABLE node_revision_new DROP INDEX uid;
ALTER TABLE node_revision_new ADD UNIQUE INDEX uniq_uid_timestamp (uid,timestamp);
INSERT IGNORE INTO node_revision_new SELECT * FROM node_revision;
DROP TABLE node_revision;
ALTER TABLE node_revision_new RENAME node_revision;

The UNIQUE INDEX uniq_uid_timestamp physically prevents (uid,timestamp) from appearing more than once. Check the contents of node_revision and make sure it is proper.
If it looks good, you could run:
DROP TABLE node_revision_backup;

or keep the backup.
If it does not look good, run this:
ALTER TABLE node_revision RENAME node_revision_old;
ALTER TABLE node_revision_backup RENAME node_revision;
DROP TABLE node_revision_old;

Give it a Try !!!
CAVEAT
You will need to check all modules involved that writes to the node_revision table.