Screen Time displays duplicate devices and suspicious activity on iPhone

If you change the Apple ID password on your device then all the devices that do not belong to you will automatically get logged out and will not have any link to your device.

Thumb Rules (Keep in mind always):

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing
6. Do not share your location with a stranger --> Find friends and share your location with Find My
7. If you see this --> What to do if you get an alert that an AirTag, Find My network accessory or set of AirPods is with you
8. Do not leave your device unlocked or without a passcode unattended in a public place --> If you think your Apple ID has been compromised

Change your Apple ID password

1. Tap Settings > your name > Password & Security.
2. Tap Change Password.
3. Enter your current password or device passcode, then enter a new password and confirm the new password. Forgot your password?
4. Tap Change or Change Password.

Change your Apple ID password - Apple Support (IN)

Sign out on your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Scroll down to the bottom.
3. Tap Sign Out.
4. Enter your Apple ID password and tap Turn Off. ... 
5. Turn on the data that you want to keep a copy of on your device.
6. Tap Sign Out.
7. Tap Sign Out again to confirm that you want to sign out of iCloud.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV ...

You can only find out if they have logged in presently, not if they have logged out.

Settings > [Tap your Name} > Device will appear here...

See the pic below

Your phone is not hacked unless you actively decided to jailbreak it. That is not something that can be done by accident or remotely, so that would be an intentional action on your part.

It is entirely possible that an account may be compromised. If that is the case for you, review the information below. Make sure you are using two factor authentication to prevent another device from logging into your account without the verification code sent to your trusted device. You can also view the devices logged into your account by going to Settings > [name on top], and scroll to the bottom. Tap on a device to remove it, and follow up by changing your Password.

If you think your Apple Account has been compromised - Apple Support

Not sure what you are referring to with 2 of your phones showing in Screen Time. A screenshot may be helpful as long as it does not contain any personal information such as your name.

• Are you using Family Sharing?
• When you go to Settings > [name on top], and scroll to the bottom, do you have devices listed there that you no longer have? If so, they were not removed from your account before you got rid of them and still logged in to your account. Tap on the device you do not want logged into your account to remove, then follow up by changing your Password.
• If you have multiple devices using your same account and don't want Screen Time to aggregate those totals, then go to Settings > Screen Time > Share Across Devices, and turn off that setting.

I would also recommend to make sure you are updated to iOS 18.5 and do a Force Restart on your device. This may take a couple of attempts to get the speed and sequence of the key presses correct.

• Press and quickly release the Volume Up button
• Press and quickly release the Volume Down button
• Press and HOLD the side button until the Apple Logo appears. Ignore anything else you see on the screen while holding the side button and it could take up to 15 seconds before you see the Apple logo.