When the pollSCM in a declarative pipeline is executed when the jobs on the same commit is queued and did not do a checkout scm the jobs is queued again on the same commit.

A test Jekninsfile:

pipeline {
    agent {
        node {
            label 'local'
        }
    }
    triggers {
        pollSCM('H/2 * * * *')
    }
    stages {
        stage('Run') {
            steps {
                checkout scm
                sh 'sleep 1000'
            }
        }
    }
}

If a new commit is pushed within in 1000 seconds and the executors on jenkins are busy jenkins will fire a new job at each pollSCM till 1 job on this commit did do a checkout. In this example this will be 5-6 jobs on the same commit.

When doing this on multiple repositories on a limited number of executors this will explode in 10000+ jobs if your unlucky. (happened to me)