Le sous-dossier contient un doublon.

Sous-dossier literally means sub-folder:

• sous- = under / sub-
• dossier = folder / file

So a sous-dossier is a folder inside another folder.

The hyphen is standard in this compound noun. French often uses a hyphen with words built with prefixes like sous- when they form a fixed noun.

It is masculine, which is why the sentence starts with Le sous-dossier.

A useful clue is the article:

• le = masculine singular
• la = feminine singular

You will also see this in the other article:

• un doublon = masculine singular

So both nouns in this sentence are masculine.

French is making the same distinction English often makes between the and a/an:

• Le sous-dossier = the subfolder
  This refers to a specific subfolder, probably one already known from the context.
• un doublon = a duplicate
  This introduces one duplicate, not a specific one already identified.

So the sentence means that the specific subfolder contains a duplicate.

Contient is the third-person singular present form of the verb contenir, which means to contain.

Conjugation in the present:

• je contiens
• tu contiens
• il / elle / on contient
• nous contenons
• vous contenez
• ils / elles contiennent

So Le sous-dossier contient... means The subfolder contains...

Because contenir is an irregular verb pattern related to tenir.

In the present tense, the il / elle / on form ends in -t:

• il tient
• il contient

This is just a conjugation pattern you need to learn. It does not follow the regular -er pattern, and the stem changes as well.

In this context, doublon means a duplicate, a duplicate entry, or something that appears twice when it should not.

Depending on context, it could refer to:

• a duplicated file
• a repeated record in a database
• a duplicate item in a list
• a photo or document that appears twice

So doublon is often used in technical, administrative, or data-related contexts.

There are other possibilities, and the best choice depends on context.

Common alternatives include:

• duplicata = a duplicate copy, often more formal
• en double = duplicated / in duplicate
• copie en double = duplicate copy
• fichier en double = duplicate file

Doublon is very common when talking about repeated data, duplicate files, or duplicate entries. It often suggests an unwanted duplicate.

A simple approximate pronunciation is:

luh soo-doh-syay kohn-tyan uh(n) doo-blohn

A few notes:

• Le sounds like luh
• sous-dossier has the stress pattern typical of French, with the flow going toward the end
• contient has a nasal sound in -tient
• doublon ends with a nasal -on

There is no strong English-style stress on one word; French is more even in rhythm.

Usually, no important liaison is made here in careful everyday speech.

For example:

• Le sous-dossier: no liaison between le and sous-dossier
• contient un: speakers generally do not make a liaison here

So you can say it smoothly without adding an extra linking consonant sound.

Because this is a straightforward subject + verb + object sentence:

• Le sous-dossier = subject
• contient = verb
• un doublon = direct object

French often uses the same basic order as English in simple statements, so this sentence maps very neatly onto The subfolder contains a duplicate.

Yes, but the meaning changes slightly.

• Le dossier = the folder
• Le sous-dossier = the subfolder

So if you say Le dossier contient un doublon, you are talking about the folder in general, not specifically a subfolder inside another folder.

The plural would be:

Les sous-dossiers contiennent des doublons.

Changes:

• Le → Les
• sous-dossier → sous-dossiers
• contient → contiennent
• un doublon → des doublons

That means The subfolders contain duplicates.

Yes, it sounds natural, especially in a technical or digital context.

A French speaker might use it for:

• file systems
• cloud storage
• database cleanup
• document organization

Depending on the exact situation, other natural alternatives could be:

• Le sous-dossier contient un fichier en double.
• Il y a un doublon dans le sous-dossier.

But your original sentence is perfectly normal and clear.