Não te enganes no ícone; eu já me enganei duas vezes hoje.

Here enganar-se is the reflexive form of enganar.

• enganar alguém = to deceive / trick someone
• enganar-se = to be mistaken / get something wrong / confuse oneself

So in this sentence, it does not mean deceive yourself literally. It means something like:

• to mistake one thing for another
• to get confused
• to click/select the wrong thing, depending on context

With ícone, the idea is basically don’t get the icon wrong or don’t mistake the icon.

They are reflexive pronouns.

• te goes with tu
• me goes with eu

Because the verb is being used as enganar-se, the pronoun is part of the verb expression:

• tu enganas-te / não te enganes
• eu enganei-me / já me enganei

For this verb, the pronoun is often essential to get the meaning be mistaken.

A quick comparison:

• Enganei o João. = I deceived João.
• Enganei-me. = I was mistaken / I got it wrong.

Because this is a negative command addressed to tu.

In Portuguese, negative commands use the present subjunctive, not the affirmative imperative form.

So:

• affirmative command: engana-te
• negative command: não te enganes

That -es ending is exactly what you expect from the present subjunctive of enganar for tu.

So:

• Não te enganes = correct
• Não te engana = not correct for this meaning

Because não forces the pronoun to come before the verb in Portuguese.

This is called proclisis.

So:

• Engana-te = affirmative command
• Não te enganes = negative command

The negation não is a very common trigger for this word order in both European and Brazilian Portuguese.

In European Portuguese, words like já often attract the clitic pronoun to come before the verb.

So:

• Já me enganei duas vezes hoje. = natural
• Enganei-me duas vezes hoje. = also natural if já is not there
• Já enganei-me... = not the standard form

So the pronoun placement here is very typical of Portugal Portuguese:

• já me enganei
• ainda me lembro
• sempre me disseram

This is one of the areas where European Portuguese clitic placement is especially important to notice.

No is a contraction of:

• em + o = no

So no ícone literally means in/on the icon, but in this kind of sentence it is better understood as:

• about the icon
• with the icon
• in choosing/identifying the icon

With enganar-se, Portuguese often uses em to mark the thing you got wrong:

• Enganei-me no nome. = I got the name wrong.
• Enganei-me no número. = I got the number wrong.
• Enganei-me no ícone. = I got the icon wrong / I mistook the icon.

Also, the article o is there because it refers to a specific icon, not just an icon in general.

Because Portuguese often uses the simple preterite where English would naturally use the present perfect.

So:

• já me enganei duas vezes hoje
  can correspond to
• I’ve already made that mistake twice today
• I’ve already got it wrong twice today

In Portuguese, enganei is completely normal even with hoje and já.

This is very important for English speakers, because English often prefers have + past participle, while Portuguese very often just uses the simple past for completed events, even if they happened today.

Já means already.

It shows that the speaker has experienced this before now, and it adds a feeling like:

• I’ve already done this
• this has happened to me before
• so I’m warning you from experience

Without já, the sentence would still be grammatical:

• Eu me enganei duas vezes hoje.

But já makes the warning feel stronger and more immediate.

No, it is not necessary.

Portuguese usually allows you to omit the subject pronoun because the verb ending already shows the person:

• (Eu) já me enganei duas vezes hoje.

Both are correct.

Here eu is probably used for one of these reasons:

• slight emphasis
• contrast with the implied tu in the first clause
• rhythm/style

So the sentence without eu would also sound natural:

• Não te enganes no ícone; já me enganei duas vezes hoje.

The semicolon links two closely related independent clauses.

The two parts are:

• Não te enganes no ícone
• eu já me enganei duas vezes hoje

They are strongly connected in meaning: the second part explains why the speaker is giving the warning.

A full stop would also be possible:

• Não te enganes no ícone. Eu já me enganei duas vezes hoje.

The semicolon just gives a slightly more polished written feel.

Yes, mainly in clitic placement and sometimes in overall style.

A Brazilian Portuguese speaker might more often say something like:

• Não se engane com o ícone; eu já me enganei duas vezes hoje.
• or use você instead of tu, depending on region

But for Portugal Portuguese, não te enganes is a very natural way to speak to someone informally using tu.

So the sentence as given is very much in line with European Portuguese usage.