dont play with same IP but different devices, even though 2 people are playing

[Tyrannical]

As for internet cafes, for the most part there's nothing wrong with playing in there. We can always check and see whether the accounts playing in a cafe (or should I say the same IP address) are owned by one person or by different people.

- The Question, how reliable is the method of investigation, how reliable the result would it be? if based on the comments I've seen is about the highlighted quoted below. 

Now is it illegal to fish under 1 IP?

It depends on the accounts or better, the 'believed to be' owner of the accounts. "There is no real way for GM's to determine who is controlling an account on another device", that's why its important that an account is distinguishable from each other. Anyone can claim that it was their brother, their sister, their friend that uses the account. A GM can't tell that. That's why it depends mostly on the account. If an account is named Xileria1 and Xileria2 and they play on 2 different devices, even different IPs, it will immediately fall under suspicion. Rarely some players are caught with it, but it does happen. 

-This is in line with my previous comment about playing in an internet cafe.
Example. I have 10 PC Clients and I have 5 Ragnarok Players. All PC clients have RO installed.

Day 1:
RO player 1 sits on PC 1, RO Player 2 sits on PC 2, RO player sits on PC3, RO Player sits on PC4, RO Player sits on PC5. PC6~10 is occupied by NON-RO players.

Day 2: 
RO player 1 and 3 is absent/not around. 
PC 2 and PC 4 , PC6~10 are occupied by NON-RO Players.
And RO Player 2,4 and 5 are present. And they are only able to sit and use on the available PC's.
RO Player 2 sits on PC1, where RO Player 1 recently used it.
RO Player 4 sits on PC 3, where RO Player 3 recently used it.
RO player 5 sits on the same PC he recently used.

Day 3~etc.

So, if we will check on the logs, the Accounts are logged will have the same IP address and have multiple Device IDs recorded over the time.
How would you guys evaluate this scenario? if basing on this statement:

"Additionally, upon checking the account history of 2 accused accounts, I found out the account was frequently logged on same device in the past. In short in the eyes of GM's, this account is most likely owned by a single player."

I just want to know because I really want to install RO in my internet cafe.

***

So what's the problem with your case? Upon my own investigation, the problem with your account is because as claire stated it was once banned for dual clienting.

- I'm not siding anyone here just to be clear, but just sharing my opinion. 
Ok, he made mistakes from the past. If we put it on real life scenario, he already atoned for it by the sanctions/penalty he received (you have given).
After that. He's clear am I right? That's why he can play again. It's up to him if he learned his lesson or not.
If he follows the rules, good. If still not, then the investigation team or individual should come up with NEW sufficient evidence and not just use the previous one.

So what do we have left ? The Logs. 
In my opinion, The Logs just shows the "TIME LOG IN " of account/character. The Online Duration and Time Log out is not there.
So what if I logged in my Account 1 at 7:03:06 AM to do fishing.
I logged out at 7:03:15 AM.
I checked on the other "shared" account 1 and logged it in at 7:03:42AM for some other matters. ( Forging, WoF, Refining, Checking items, etc)

• Records will show that My account 1 is logged in at 7:03:06 AM
• Records will show that Shared Account 1 is logged in at 7:03:42 AM
  All under the same IP and Device ID.
- initial presumption would be? Dual Client? Right? But no. 

The source is already unreliable because of the lack of information shown.
So I suggest to to update the Account Information History on the website as well (duration online, and log out time to be included)
This will help the GMs/Staffs to solve this kind of issues easier and will have a more reliable source of information.

Good day everyone.

My comment is inside the quoted message.