There are many ways you can express this, and you can change how you respond to this situation, depending on who you're talking to. The first example acknowledges that they do look very much alike but they are not related, so they obviously cannot be twins.
The second response uses the word "even" which shows that most people would think they must be related (as in twins) but the truth is contrary to what it seems. You provide an explanation in the latter part of the sentence by explaining that they are merely friends.
They look like twins but they aren't related. Just friends.
They look so much alike people think they are twins but they are just friends.
Some people think those girls are twins but they aren't even related.
They look like twins but they aren't related. Just friends.
They look so much alike people think they are twins but they are just friends.
Some people think those girls are twins but they aren't even related.
No, they aren't twins, just close friends.
If they weren't just friends, people would think they were twins.
It's amazing how they look so much alike but aren't related at all.
They look like twins but they aren't related. Just friends.（双子に見えますけど、血のつながりはありません。ただの友達です）
They look so much alike people think they are twins but they are just friends.（すごく似ているので、みんな双子だと思うんですけど、ただの友達なんです）
Some people think those girls are twins but they aren't even related.（双子だと思う人もいますけど、血のつながりもないんです）
No, they aren't twins, just close friends.（いいえ、双子じゃありません、ただの親しい友達です）
If they weren't just friends, people would think they were twins.（ただの友達じゃなかったとしたら、みんな双子だと思うだろうね）
It's amazing how they look so much alike but aren't related at all.（あんなに似ていて血のつながりがないのは驚きです）
The look like they're related but they aren't. They're just friends.
Some make mistake them as twins but they are only friends
You probably think they are twins/related but they aren't.
If you are having a conversation and need to clarify that two people are not twins or even related then some common expressions could be as per below;
I know you probably think they are twins, but they are actually just friends.
or
So many people think that they are twins but they are just close friends.
or
I see the similarities of how some people might think that they are related or twins but they are only friends.
二人の人が双子、あるいは血のつながりがないことを伝えたいなら、以下の表現がよく使われます。
I know you probably think they are twins, but they are actually just friends.
「たぶん双子と思うだろうけど、ただの友達だよ」
So many people think that they are twins but they are just close friends.
「双子だと思う人が多いけど、ただの親しい友達だよ」
I see the similarities of how some people might think that they are related or twins but they are only friends.
「あの二人は確かによく似ていますけど、ただの友達なんです」
They look like twins but are actually only friends
They are just friends but look like twins
Twins are brothers/sisters that are born from the same mother and father and around the same time and look like each other
so if two people look like each other (twins) but are only friends then you would say 'they are just friends but look like twins'
twins'（双子）とは、同じ両親からほぼ同じ時間に生まれたよく似たきょうだいをいいます。
ですから、二人の人がよく似ているけども、ただの友達なら、次のように言えます。
'they are just friends but look like twins'
（彼らはただの友達ですが双子みたいに見えます）
They look so much alike I thought they were brothers.
I assumed they were brothers.
When you assume something, you believe it to be true automatically without checking.
There is also a word that is used in English that was borrowed from the German language "Doppelgänger". This is a commonly used word that means someone who looks just like someone else. You could say something like:
"One is the other's doppelgänger."
"assume" は、確認することなくすぐに物事を事実と信じることを言います。
また、"Doppelgänger" というドイツ語からの借用語もあります。これは「そっくりな人」という意味で、英語でよく使われます。
次のように言えます。
"One is the other's doppelgänger."
（～は～のドッペルゲンガーだ）
They are similar looking, but they're just friends.
To tell someone that two people look like twins, but are actually just friends, you can do so in the following ways:
"They look identical, but they're just friends."
"They are similar looking, but they're just friends."
二人の人について「双子みたいだけどただの友達」と説明したいなら、次のように言えます。
"They look identical, but they're just friends."（そっくりだけどただの友達です）
"They are similar looking, but they're just friends."（よく似ているけどただの友達です）
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