Steps to reproduce:
0. Reproduced with MS SQLServer 2008R2 and MySQL 5.6. Probably, a database independent issue.
1. In 5.2.8, add a Wiki Portlet to a page.
Set permissions.user.check.algorithm=6 so it doesn't have to be migrated later.
2. Add content to a page.
3. Edit the page several times in quick succession.
Verify in the database table "WikiPage" that there are modifiedDate timestamps that are the same. If not, do step 3 faster.
4. Backup and upgrade to 6.2.
Result: In the middle of the upgrade, this error occurs.
23:31:04,667 INFO [localhost-startStop-1][UpgradeProcess:175] Upgrading com.liferay.portal.upgrade.v6_2_0.UpgradeSocial
23:31:04,808 WARN [localhost-startStop-1][UpgradeSocial:79] Unable to add activity 10217
java.sql.SQLException: Cannot insert duplicate key row in object 'dbo.SocialActivity' with unique index 'IX_8F32DEC9'.