seikyuukikan ga nagai ue ni, nizyuuseikyuu mo aru.

A natural reading is:

せいきゅうきかん が ながい うえ に、にじゅうせいきゅう も ある。

• 請求期間 = せいきゅうきかん
• 長い = ながい
• うえに = うえに
• 二重請求 = にじゅうせいきゅう
• ある = ある

うえに means something like on top of that, in addition, or what’s more.

It connects one negative or positive point to another point of the same general kind. In this sentence, it adds another complaint:

• 請求期間が長い = the billing period is long
• うえに、二重請求もある = and on top of that, there are also double charges

So the overall feeling is: Not only is the billing period long, but there are also duplicate charges.

Because うえに attaches to the plain form of the word before it.

For an い-adjective, that means using the normal dictionary form:

• 長い + うえに → 長いうえに

You do not use the adverbial -く form here.

Compare:

• 高いうえに = on top of being expensive
• 難しいうえに = on top of being difficult

For a na-adjective or noun, you usually use である / な / で patterns depending on style, but with an い-adjective like 長い, the plain form is correct.

請求期間が長い literally means the billing period is long.

Here, が marks 請求期間 as the thing that has the property 長い.

This is very natural with adjective sentences:

• 時間が長い = the time is long
• 値段が高い = the price is high

If you used は instead, it would sound more like setting up a topic:

• 請求期間は長い = as for the billing period, it is long

That is also possible in some contexts, but が is very normal when simply stating one problem before adding another with うえに.

も means also / too.

So:

• 二重請求がある = there are duplicate charges
• 二重請求もある = there are duplicate charges too / as well

The も helps match the meaning of うえに. The speaker is listing multiple problems:

1. the billing period is long
2. there are also duplicate charges

So も strengthens the sense of another bad thing in addition.

ある is the verb used for the existence of inanimate things or abstract things.

Here, 二重請求 is not a person or animal, so ある is used:

• 二重請求がある = there are duplicate charges

This is similar to:

• 問題がある = there is a problem
• ミスがある = there is a mistake
• 不備がある = there is a defect / issue

So ある here means to exist / to occur / to be present.

二重請求 is a compound noun:

• 二重 = double, duplicate
• 請求 = billing, charge, claim for payment

Together, 二重請求 means double billing, duplicate billing, or being charged twice.

Japanese often combines nouns directly like this without any extra word in between.

Yes. Even if the grammar itself is neutral, the sentence clearly sounds like a complaint or criticism.

That is because both parts describe problems:

• 請求期間が長い = the billing period is long
• 二重請求もある = there are duplicate charges too

Also, うえに often feels especially natural when piling up reasons, often in complaints:

• 値段が高いうえに、質も悪い。
  It’s expensive, and on top of that, the quality is bad.

So this sentence has a strong “and that’s not the only problem” feeling.

Yes, that is a very natural translation.

A good English rendering is:

Not only is the billing period long, but there are also duplicate charges.

That said, うえに is not always exactly the same as the English pattern not only A, but also B in a strict grammatical sense. It often feels more like:

• on top of that
• in addition
• what’s more

But in many cases, not only ... but also ... is an excellent translation.

They can all connect ideas, but the nuance is a little different.

• うえに = on top of that, often used when adding another point of the same type
• し = softer, looser listing of reasons
• だけでなく = not only ... but also ..., a bit more explicit and structured

For example:

• 請求期間が長いうえに、二重請求もある。
  On top of the billing period being long, there are also duplicate charges.

• 請求期間が長いし、二重請求もある。
  The billing period is long, and there are also duplicate charges.

• 請求期間が長いだけでなく、二重請求もある。
  Not only is the billing period long, but there are also duplicate charges.

All are possible, but うえに has a strong sense of adding another troublesome point.

The sentence breaks down like this:

請求期間 が 長い
= the billing period is long

うえに
= on top of that / in addition

二重請求 も ある
= there are also duplicate charges

So the structure is:

[Problem 1] + うえに + [Problem 2]

That is a very common pattern in Japanese.

Yes, very possibly. Japanese often leaves the broader subject unstated when it is obvious from context.

For example, in a longer conversation this could mean:

• This company’s billing period is long, and they also do double billing.
• This plan has a long billing period, and there are duplicate charges too.

The sentence itself only explicitly states the two problems. The larger topic is probably understood from context.