Mac said another device was using computer's IP address?

It was a message that popped up right after I brought it back from sleep mode. I clicked OK and then the connection eventually worked again. During this time my sister came to visit with her iPhone and there are other lap tops and another iPhone that my family uses as well. I've just never had the problem before so was wondering if that was the issue? Is there anything to do? Internet has worked fine since but it was still a bit alarming seeing that message.

Added (1). I have no idea what "your computer is wide open" means…

Added (2). Also, I've had no issue with my signal velocity… If that means the speed of my 'net.

Your computer is wide open so I suggest you code your modem your router and your computer. Fewer intruders will improve your signal velocity a lot.

What it means is, another device connected to your local area network/router
has the same private IP address.
It's either a mistake from the device or the router.
What you can try is rebooting the device, rebooting the router.

Wow! This Question is great, I also waiting for best answer

Your (pretty much all) wireless router uses DHCP to allocate IP addresses to local devices (laptops, tablets, phones,.). The allocation has a expiration (called a lease time), after which the router will reuse the allocated IP address. What probably happened is your Mac's IP allocation timed out while it was asleep, and your sisters device (legally) was allocated the address. There's a protocol of reacquiring IPs between routers and devices that usually just happens without error notices. In this case your Mac's OS decided to alert you in case this was malicious attempt.