This means that you have a modsecurity rule with the same id. All of our rules have unique id's. This error can only occur for one of two reasons:
1) You have loaded our rules twice. If you are using ASL or aum, this means you have a third party product that is managing modsecurity. Disable any third part software.
If you are not using ASL or aum, please follow the installation guide and ensure that you have modsecurity configured as described in that article document. These installation procedures will only load the rules once.
2) You have rules that are using id's already assigned to other rules.
If you are using third party or custom rules, check to make sure they have unique id's. Modsecurity requires a unique id for each rule. The range 300000-399999 is reserved for our rules, do not use this range for any custom rules, and if you have third party rules with these id's be sure to remove these rules.