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 Atomicorp software to manage modsecurity (AP, AWP, ASL or aum) this means you have a third party product that is also managing, configuring and/or modifying modsecurity or your web servers configuration. Disable any third party modsecurity software.
If you are not using Atomicorp software to manage modsecurity please follow the installation guide and ensure that you have modsecurity configured as described in that article. These installation procedures will only load our 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 or change the rule ids to an unassigned range.