diff --git a/caddy.go b/caddy.go
index 628673a73..917a5d069 100644
--- a/caddy.go
+++ b/caddy.go
@@ -612,7 +612,7 @@ func ValidateAndExecuteDirectives(cdyfile Input, inst *Instance, justValidate bo
 
 	sblocks, err = inst.context.InspectServerBlocks(cdyfile.Path(), sblocks)
 	if err != nil {
-		return err
+		return fmt.Errorf("error inspecting server blocks: %v", err)
 	}
 
 	return executeDirectives(inst, cdyfile.Path(), stype.Directives(), sblocks, justValidate)
diff --git a/caddyhttp/httpserver/plugin.go b/caddyhttp/httpserver/plugin.go
index ea31a58d8..93811abcb 100644
--- a/caddyhttp/httpserver/plugin.go
+++ b/caddyhttp/httpserver/plugin.go
@@ -117,12 +117,14 @@ func (h *httpContext) saveConfig(key string, cfg *SiteConfig) {
 // executing directives and otherwise prepares the directives to
 // be parsed and executed.
 func (h *httpContext) InspectServerBlocks(sourceFile string, serverBlocks []caddyfile.ServerBlock) ([]caddyfile.ServerBlock, error) {
+	siteAddrs := make(map[string]string)
+
 	// For each address in each server block, make a new config
 	for _, sb := range serverBlocks {
 		for _, key := range sb.Keys {
 			key = strings.ToLower(key)
 			if _, dup := h.keysToSiteConfigs[key]; dup {
-				return serverBlocks, fmt.Errorf("duplicate site address: %s", key)
+				return serverBlocks, fmt.Errorf("duplicate site key: %s", key)
 			}
 			addr, err := standardizeAddress(key)
 			if err != nil {
@@ -138,6 +140,23 @@ func (h *httpContext) InspectServerBlocks(sourceFile string, serverBlocks []cadd
 				addr.Port = Port
 			}
 
+			// Make sure the adjusted site address is distinct
+			addrCopy := addr // make copy so we don't disturb the original, carefully-parsed address struct
+			if addrCopy.Port == "" && Port == DefaultPort {
+				addrCopy.Port = Port
+			}
+			addrStr := strings.ToLower(addrCopy.String())
+			if otherSiteKey, dup := siteAddrs[addrStr]; dup {
+				err := fmt.Errorf("duplicate site address: %s", addrStr)
+				if (addrCopy.Host == Host && Host != DefaultHost) ||
+					(addrCopy.Port == Port && Port != DefaultPort) {
+					err = fmt.Errorf("site defined as %s is a duplicate of %s because of modified "+
+						"default host and/or port values (usually via -host or -port flags)", key, otherSiteKey)
+				}
+				return serverBlocks, err
+			}
+			siteAddrs[addrStr] = key
+
 			// If default HTTP or HTTPS ports have been customized,
 			// make sure the ACME challenge ports match
 			var altHTTPPort, altTLSSNIPort string
diff --git a/caddyhttp/httpserver/plugin_test.go b/caddyhttp/httpserver/plugin_test.go
index 5a60f2e83..f7b9cfc00 100644
--- a/caddyhttp/httpserver/plugin_test.go
+++ b/caddyhttp/httpserver/plugin_test.go
@@ -153,6 +153,23 @@ func TestInspectServerBlocksWithCustomDefaultPort(t *testing.T) {
 	}
 }
 
+// See discussion on PR #2015
+func TestInspectServerBlocksWithAdjustedAddress(t *testing.T) {
+	Port = DefaultPort
+	Host = "example.com"
+	filename := "Testfile"
+	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)
+	input := strings.NewReader("example.com {\n}\n:2015 {\n}")
+	sblocks, err := caddyfile.Parse(filename, input, nil)
+	if err != nil {
+		t.Fatalf("Expected no error setting up test, got: %v", err)
+	}
+	_, err = ctx.InspectServerBlocks(filename, sblocks)
+	if err == nil {
+		t.Fatalf("Expected an error because site definitions should overlap, got: %v", err)
+	}
+}
+
 func TestInspectServerBlocksCaseInsensitiveKey(t *testing.T) {
 	filename := "Testfile"
 	ctx := newContext(&caddy.Instance{Storage: make(map[interface{}]interface{})}).(*httpContext)