diff --git a/openwisp-config/files/lib/openwisp/utils.lua b/openwisp-config/files/lib/openwisp/utils.lua
index 82db0ea..ac98754 100644
--- a/openwisp-config/files/lib/openwisp/utils.lua
+++ b/openwisp-config/files/lib/openwisp/utils.lua
@@ -7,6 +7,13 @@ function starts_with_dot(str)
     return false
 end
 
+function add_values_to_set(set, values)
+    for i, el in pairs(values) do
+        set[el] = true
+    end
+    return set
+end
+
 -- writes uci section, eg:
 --
 -- write_uci_section(cursor, 'network', {
@@ -49,9 +56,12 @@ function write_uci_option(cursor, config, name, key, value)
     if type(value) == 'table' then
         -- create set with unique values
         local set = {}
-        for i, el in pairs(value) do
-            set[el] = true
+        -- read existing value
+        current = cursor:get(config, name, key)
+        if type(current) == 'table' then
+            set = add_values_to_set(set, current)
         end
+        set = add_values_to_set(set, value)
         -- reset value var with set contents
         value = {}
         for item_value, present in pairs(set) do
diff --git a/openwisp-config/tests/test_utils.lua b/openwisp-config/tests/test_utils.lua
index 269fde0..266ba95 100644
--- a/openwisp-config/tests/test_utils.lua
+++ b/openwisp-config/tests/test_utils.lua
@@ -138,4 +138,95 @@ function TestUtils.test_is_table_empty_false()
         el = "value"
     }), false)
 end
+
+function TestUtils.test_merge_uci_option()
+    u = uci.cursor(write_dir)
+    -- prepare config
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        ipaddr = "172.27.254.252/16",
+        proto = "static",
+        ifname = "wlan1"
+    })
+    u:commit('network')
+    -- add one option
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        added = "merged"
+    })
+    u:commit('network')
+    local file = io.open(write_dir..'/network')
+    luaunit.assertNotNil(file)
+    local contents = file:read('*all')
+    luaunit.assertNotNil(string.find(contents, "config interface 'wlan1'"))
+    luaunit.assertNotNil(string.find(contents, "option ifname 'wlan1'"))
+    luaunit.assertNotNil(string.find(contents, "option proto 'static'"))
+    luaunit.assertNotNil(string.find(contents, "option ipaddr '172.27.254.252/16'"))
+    luaunit.assertNotNil(string.find(contents, "option added 'merged'"))
+end
+
+function TestUtils.test_merge_uci_list()
+    u = uci.cursor(write_dir)
+    -- prepare config
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        ipaddr = {"172.27.254.252/16"},
+        proto = "static",
+        ifname = "wlan1"
+    })
+    u:commit('network')
+    -- add one option
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        ipaddr = {"172.27.254.253/16"},
+    })
+    u:commit('network')
+    local file = io.open(write_dir..'/network')
+    luaunit.assertNotNil(file)
+    local contents = file:read('*all')
+    luaunit.assertNotNil(string.find(contents, "list ipaddr '172.27.254.252/16'"))
+    luaunit.assertNotNil(string.find(contents, "list ipaddr '172.27.254.253/16'"))
+end
+
+function TestUtils.test_merge_uci_list_duplicate()
+    u = uci.cursor(write_dir)
+    -- prepare config
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        ipaddr = {"172.27.254.252/16"},
+        proto = "static",
+        ifname = "wlan1"
+    })
+    u:commit('network')
+    -- add one option
+    write_uci_section(u, 'network', {
+        [".name"] = "wlan1",
+        [".type"] = "interface",
+        [".anonymous"] = false,
+        [".index"] = 5,
+        ipaddr = {"172.27.254.252/16"},
+    })
+    u:commit('network')
+    local file = io.open(write_dir..'/network')
+    luaunit.assertNotNil(file)
+    local contents = file:read('*all')
+    local _, count = string.gsub(contents, "list ipaddr '172.27.254.252/16'", "")
+    luaunit.assertEquals(count, 1)
+end
+
 os.exit(luaunit.LuaUnit.run())