Turn Conversation View on and off
Grouping your emails into conversations, based on subject line, is a Gmail feature. You can turn it on/off in your Gmail settings. Click the gear icon in the top right corner, then click Settings. Scroll til you see Conversation view, and click the button text to Conversation view off.
Note: Turning conversation view off will prevent reminders from working properly. Without conversation view, the lightning bolt tracking indicators will only attach themselves to the first message.