Avoid writing duplicate index URLs with --emit-index-url (#8226)

closes #8116

---------

Co-authored-by: Charlie Marsh <[email protected]>

Author: Aditya-PS-05

crates/uv/src/commands/pip/compile.rs

@@ -4,6 +4,7 @@ use std::path::Path;
 use anyhow::{anyhow, Result};
 use itertools::Itertools;
 use owo_colors::OwoColorize;
+use rustc_hash::FxHashSet;
 use tracing::debug;
 
 use uv_cache::Cache;
@@ -466,9 +467,12 @@ pub(crate) async fn pip_compile(
             writeln!(writer, "--index-url {}", index.url().verbatim())?;
             wrote_preamble = true;
         }
+        let mut seen = FxHashSet::default();
         for extra_index in index_locations.implicit_indexes() {
-            writeln!(writer, "--extra-index-url {}", extra_index.url().verbatim())?;
-            wrote_preamble = true;
+            if seen.insert(extra_index.url()) {
+                writeln!(writer, "--extra-index-url {}", extra_index.url().verbatim())?;
+                wrote_preamble = true;
+            }
         }
     }
 

crates/uv/tests/it/pip_compile.rs

@@ -5263,7 +5263,8 @@ fn emit_index_urls() -> Result<()> {
             .arg("--index-url")
             .arg("https://test.pypi.org/simple/")
             .arg("--extra-index-url")
-            .arg("https://pypi.org/simple"), @r###"
+            .arg("https://pypi.org/simple")
+            .env("UV_EXTRA_INDEX_URL", "https://pypi.org/simple"), @r###"
     success: true
     exit_code: 0
     ----- stdout -----