feat(config): validate duplicate model names

Add validation to ensure model_name is unique across all entries in
model_list. This prevents potential conflicts when multiple model
configs share the same model_name identifier.

pkg/config/config.go

@@ -294,6 +294,11 @@ func LoadConfig(path string) (*Config, error) {
 		cfg.ModelList = ConvertProvidersToModelList(cfg)
 	}
 
+	// Validate model_list for uniqueness and required fields
+	if err := cfg.ValidateModelList(); err != nil {
+		return nil, err
+	}
+
 	return cfg, nil
 }
 
@@ -471,12 +476,20 @@ func (c *Config) HasProvidersConfig() bool {
 }
 
 // ValidateModelList validates all ModelConfig entries in the model_list.
-// It checks that each model_name/model combination is valid.
+// It checks that each model_name/model combination is valid and that
+// model_name is unique across all entries.
 func (c *Config) ValidateModelList() error {
+	seen := make(map[string]int)
 	for i := range c.ModelList {
 		if err := c.ModelList[i].Validate(); err != nil {
 			return fmt.Errorf("model_list[%d]: %w", i, err)
 		}
+		// Check for duplicate model_name
+		name := c.ModelList[i].ModelName
+		if prevIdx, exists := seen[name]; exists {
+			return fmt.Errorf("model_list: duplicate model_name %q at index %d and %d", name, prevIdx, i)
+		}
+		seen[name] = i
 	}
 	return nil
 }

pkg/config/model_config_test.go

@@ -6,6 +6,7 @@
 package config
 
 import (
+	"strings"
 	"sync"
 	"testing"
 )
@@ -163,6 +164,7 @@ func TestConfig_ValidateModelList(t *testing.T) {
 		name    string
 		config  *Config
 		wantErr bool
+		errMsg  string // partial error message to check
 	}{
 		{
 			name: "valid list",
@@ -183,6 +185,7 @@ func TestConfig_ValidateModelList(t *testing.T) {
 				},
 			},
 			wantErr: true,
+			errMsg:  "model_name is required",
 		},
 		{
 			name: "empty list",
@@ -191,6 +194,29 @@ func TestConfig_ValidateModelList(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name: "duplicate model_name",
+			config: &Config{
+				ModelList: []ModelConfig{
+					{ModelName: "gpt-4", Model: "openai/gpt-4o", APIKey: "key1"},
+					{ModelName: "gpt-4", Model: "openai/gpt-4-turbo", APIKey: "key2"},
+				},
+			},
+			wantErr: true,
+			errMsg:  "duplicate model_name",
+		},
+		{
+			name: "duplicate model_name non-adjacent",
+			config: &Config{
+				ModelList: []ModelConfig{
+					{ModelName: "model-a", Model: "openai/gpt-4o"},
+					{ModelName: "model-b", Model: "anthropic/claude"},
+					{ModelName: "model-a", Model: "openai/gpt-4-turbo"},
+				},
+			},
+			wantErr: true,
+			errMsg:  "duplicate model_name \"model-a\"",
+		},
 	}
 
 	for _, tt := range tests {
@@ -199,6 +225,11 @@ func TestConfig_ValidateModelList(t *testing.T) {
 			if (err != nil) != tt.wantErr {
 				t.Errorf("ValidateModelList() error = %v, wantErr %v", err, tt.wantErr)
 			}
+			if err != nil && tt.errMsg != "" {
+				if !strings.Contains(err.Error(), tt.errMsg) {
+					t.Errorf("ValidateModelList() error = %v, want error containing %q", err, tt.errMsg)
+				}
+			}
 		})
 	}
 }