Double error message when referencing variable of type union of string literals from ambient declaration

### 🔎 Search Terms

double error, same message twice, ambient declaration, string literal union

### 🕗 Version & Regression Information

This changed between versions 3.9.7 and 4.0.5

### ⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/CYUwxgNghgTiAEYD2A7AzgF3gDwFzwCIoD4AfQgIwIChrl0sBPfFAVwFsKQZ4BeHIA

### 💻 Code

```ts
declare const x: "a" | "b"

const y: number = x
// ~
// Type 'string' is not assignable to type 'number'.
// Type 'string' is not assignable to type 'number'. (2322)
```

### 🙁 Actual behavior

You see same error message twice

### 🙂 Expected behavior

The error message should be the one in 3.9.7 ie...
```
Type '"a" | "b"' is not assignable to type 'number'.
Type '"a"' is not assignable to type 'number'. (2322)
```

Or the following message just once also works...
```
Type 'string' is not assignable to type 'number'. (2322)
```

### Additional information about the issue

_No response_

Start with the provided TypeScript Playground reproduction and compare its diagnostic with the stated 3.9.7 and 4.0.5 behavior. Trace the compiler's type-checking diagnostic path for assigning the ambient string-literal union to number; done means the duplicate message is removed and the diagnostic matches one of the expected forms.

Written by the indexing model from the issue text.