You haven't specified the Android OS version nor the Gmail app version, but since the settings are pretty much straightforward, I'll leave here an overview covering the basics for all available versions.
Summary
Three things might be happening:
- You are receiving notifications for each email that arrives;
- You have the "notify once" active and are receiving notifications for the email that arrived, but also for a label you've setup to notify;
- You are receiving notifications from the Gmail app, and from another app where you've also configured the same account.
Below is a summary for the Gmail app settings by version. Try and adjust them to fit your needs.
If you continue to experience the same issue, you're probably facing a bug and the best option is to report it.
Android 4.0.x (Ice cream sandwich)
To change Gmail settings, touch "Menu" > "Settings" from any Gmail screen.
Under "Account settings", you will find:
Email notifications:
When checked, you receive a notification when you have new email.
Ringtone:
Available when "Email notifications" is selected.
Touch to change any of three settings: The ringtone and vibrate settings for notifications, and an option to notify only the first time you have new email since you last read your messages, rather than each time a new message arrives.
Official Gmail app support page by Google for this Android version.
Android 3.0.x (Honeycomb)
To change Gmail settings, touch the Menu button at the top-right of the Application bar and touch Settings.
Under "Account settings", you will find:
Email notifications
When checked, you receive a notification when you have new email.
Notify once
Check to receive a notification from Gmail only the first time you have new email since you last read your messages, rather than each time a new message arrives.
Official Gmail app support page by Google for this Android version.
Android 2.3.x (Gingerbread)
To access the Gmail app's Account settings menu:
- Open Gmail.
- Press Menu.
- Select More > Settings > Select account
Under "Notification settings", you will find:
Official Gmail app support page by Google for this Android version.