How to clear the results in Aggregate Report's table in JMeter automatically when executing the same test plan again?

Jmeter does not have a flag to do this automatically.

Generally, you have to clear the results with CTRL+E (in windows) or from the file menu, under RUN > CLEAR or RUN > CLEAR ALL

You might be able to write a beanshell script to clear the results everytime you execute the script.

http://www.javadocexamples.com/java_examples/org/apache/jmeter/samplers/SampleListener/

Here is a Beanshell script that will clear the results everytime you execute it:

import org.apache.jmeter.gui.GuiPackage;
import org.apache.jmeter.gui.JMeterGUIComponent;
import org.apache.jmeter.gui.tree.JMeterTreeNode;
import org.apache.jmeter.samplers.Clearable;

log.info("Clearing All ...");

guiPackage = GuiPackage.getInstance();

guiPackage.getMainFrame().clearData();
for (JMeterTreeNode node : guiPackage.getTreeModel().getNodesOfType(Clearable.class)) {
    JMeterGUIComponent guiComp = guiPackage.getGui(node.getTestElement());
    if (guiComp instanceof Clearable){
        Clearable item = (Clearable) guiComp;
        try {
            item.clearData();
        } catch (Exception ex) {
            log.error("Can't clear: "+node+" "+guiComp, ex);
        }
    }
}

To use this Beanshell script in your JMeter Script :

1) Select the root node of your JMeter Script, and, using the mouse menu, add a setup node :

   Add / Threads (Users) / setup Thread Group

2) Select the newly created node, and using the mouse menu, add a script node :

   Add / Samplers / Beanshell Sampler

3) Finally, copy and paste the above script into the Script window.

Note: Manual procedure to achieve this via GUI functions is given below.

The Clear option is available under the Run menu.