Correct

常 vs 常常

/ cháng vs chángcháng /
Strokes

Formula

1
常来常往 (frequently come and go)
2
常备不懈 (always be on the alert)

Definition

常 and 常常 both are Adverbs, which means [often; frequently].

Example

常 and 常常 both words indicate that an action, behaviour or state occurs repeatedly with short intervals.
The two words are mostly interchangeable.
1
cháng
  
(
chángcháng
常常
  
)
lín
邻居
men
láiwǎng
来往
  

He and his neighbors visited each other frequently.

2
Lǎowáng
老王
cháng
  
(
chángcháng
常常
  
)
dào
jiāo
郊区
diào
钓鱼
  

Lao Wang often goes to the suburb to fish.

In written language, if it is followed by a monosyllabic word, 常 is mostly used.
3
Huánjìng
环境
cháng
suí
shíjiān
时间
de
biànhuà
变化
ér
biànhuà
变化
  

Environment always change with the time.

4
Zhīcháng
知足常乐
shì
chángshòu
长寿
de
jué
秘诀
zhī
之一
  

To be content and happy about one's life is also one of the secrets of his longevity.

The negative form is mostly 不常.
5
zuìjìn
最近
cháng
lái
le
  

He seldom showed up recently.

6
Lǎotàitai
老太太
rén
zhù
  
,
ér
儿女
cháng
huíjiā
回家
  

The old lady lives alone, and her children rarely visit her.

In four-character phrases, 常 is used and not 常常, e.g. 常来常往 (frequently come and go), 常备不懈 (always be on the alert), etc.
常来常往 (frequently come and go) 常备不懈 (always be on the alert)
7
rènshi
认识
  
,
dàn
liǎ
bìng
cháng
lái
cháng
wǎng
  

I know him, but we dont frequently see each other.

8
突击
xúnchá
巡察
yǒuzhù
有助于
shǐ
使
yuángōng
员工
chángbèixiè
常备不懈
  

Surprise visits help to keep the staff on their toes.