diff --git a/docs/configuration.md b/docs/configuration.md
index d447045f..93af3cf4 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -334,7 +334,7 @@ imgproxy can send logs to syslog, but this feature is disabled by default. To en
 
 ## Miscellaneous
 
-* `IMGPROXY_BASE_URL`: base URL prefix that will be added to every requested image URL. For example, if the base URL is `http://example.com/images` and `/path/to/image.png` is requested, imgproxy will download the source image from `http://example.com/images/path/to/image.png`. Default: blank.
+* `IMGPROXY_BASE_URL`: base URL prefix that will be added to every requested image URL. For example, if the base URL is `http://example.com/images` and `/path/to/image.png` is requested, imgproxy will download the source image from `http://example.com/images/path/to/image.png`. If the image URL already contains the prefix, it won't be added. Default: blank.
 * `IMGPROXY_USE_LINEAR_COLORSPACE`: when `true`, imgproxy will process images in linear colorspace. This will slow down processing. Note that images won't be fully processed in linear colorspace while shrink-on-load is enabled (see below).
 * `IMGPROXY_DISABLE_SHRINK_ON_LOAD`: when `true`, disables shrink-on-load for JPEG and WebP. Allows to process the whole image in linear colorspace but dramatically slows down resizing and increases memory usage when working with large images.
 * `IMGPROXY_STRIP_METADATA`: when `true`, imgproxy will strip all metadata (EXIF, IPTC, etc.) from JPEG and WebP output images. Default: `true`.
diff --git a/processing_options.go b/processing_options.go
index bbd47f3a..1bf3b575 100644
--- a/processing_options.go
+++ b/processing_options.go
@@ -304,6 +304,14 @@ func colorFromHex(hexcolor string) (rgbColor, error) {
 	return c, nil
 }
 
+func addBaseURL(u string) string {
+	if len(conf.BaseURL) == 0 || strings.HasPrefix(u, conf.BaseURL) {
+		return u
+	}
+
+	return fmt.Sprintf("%s%s", conf.BaseURL, u)
+}
+
 func decodeBase64URL(parts []string) (string, string, error) {
 	var format string
 
@@ -327,9 +335,7 @@ func decodeBase64URL(parts []string) (string, string, error) {
 		return "", "", fmt.Errorf("Invalid url encoding: %s", encoded)
 	}
 
-	fullURL := fmt.Sprintf("%s%s", conf.BaseURL, string(imageURL))
-
-	return fullURL, format, nil
+	return addBaseURL(string(imageURL)), format, nil
 }
 
 func decodePlainURL(parts []string) (string, string, error) {
@@ -355,9 +361,7 @@ func decodePlainURL(parts []string) (string, string, error) {
 		return "", "", fmt.Errorf("Invalid url encoding: %s", encoded)
 	}
 
-	fullURL := fmt.Sprintf("%s%s", conf.BaseURL, unescaped)
-
-	return fullURL, format, nil
+	return addBaseURL(unescaped), format, nil
 }
 
 func decodeURL(parts []string) (string, string, error) {