fix(cortex-common): warn on duplicate -c config flags

Fixes bounty issue #1628

When the user specifies the same config key multiple times using the -c
flag (e.g., cortex run -c model=gpt-4 -c model=claude), the CLI now emits
a warning to stderr indicating which key is duplicated and which value
will be used. Previously, duplicates were silently overridden.

Author: factorydroid

cortex-common/src/config_override.rs

@@ -1,5 +1,6 @@
 //! Configuration override helpers for CLI.
 
+use std::collections::HashSet;
 use std::path::PathBuf;
 
 use cortex_protocol::AskForApproval;
@@ -18,8 +19,13 @@ pub struct CliConfigOverrides {
 
 impl CliConfigOverrides {
     /// Parse the raw overrides into key-value pairs.
+    ///
+    /// If a key is specified multiple times, a warning is emitted to stderr
+    /// and the last value is used.
     pub fn parse_overrides(&self) -> Result<Vec<(String, toml::Value)>, String> {
         let mut result = Vec::new();
+        let mut seen_keys: HashSet<String> = HashSet::new();
+
         for raw in &self.raw_overrides {
             let parts: Vec<&str> = raw.splitn(2, '=').collect();
             if parts.len() != 2 {
@@ -30,6 +36,15 @@ impl CliConfigOverrides {
             let key = parts[0].trim().to_string();
             let value_str = parts[1].trim();
 
+            // Warn if key was already specified
+            if seen_keys.contains(&key) {
+                eprintln!(
+                    "Warning: config key '{}' specified multiple times, using last value '{}'",
+                    key, value_str
+                );
+            }
+            seen_keys.insert(key.clone());
+
             // Try to parse as various TOML types
             let value = if value_str.eq_ignore_ascii_case("true") {
                 toml::Value::Boolean(true)