添加缺少的行，但不更改日期

我正在尝试将缺失的行添加到数据帧中（在每个NO_REF值之内），同时在某些列上进行线性插值，并在另一些列上插入最后一个非NA值。当间隔之后的DATE_X值大于间隔之前的最后一个DATE值时，我无法弄清楚如何防止插入缺失的日期。

这是我的数据框：

df = data.frame(DATE = as.Date(c("2016-01-31","2016-03-31","2016-05-31","2016-08-31","2016-12-31","2016-02-29","2016-04-30","2016-06-30","2016-08-31","2016-10-31","2016-12-31","2015-01-31","2015-02-28","2015-06-30","2015-10-31","2015-12-31")), 
            DATE_X = as.Date(c("2010-01-31","2010-01-31","2016-04-30","2015-03-31","2015-03-31","2010-10-31","2010-10-31","2016-05-31","2016-05-31","2015-07-31","2015-07-31","2013-01-31","2013-01-31","2013-01-31","2015-09-30","2015-09-30")),
            NO_REF = c("P1","P1","P1","P2","P2","O1","O1","O1","O1","R1","R2","Q1","Q1","Q1","Q1","Q1"),
            KAP = as.double(15:30),
            DIV =c("PI","PI","PI","PI","PI","OP","OP","OP","OP","PR","PR","OP","OP","OP","OP","OP"))

这是我的代码：

library(dplyr)
library(multidplyr)
library(zoo)

cluster <- create_cluster(3)
cluster_eval(cluster,library(dplyr))
cluster_eval(cluster,library(zoo))

result = df %>% partition(NO_REF,cluster=cluster) %>%
group_by(NO_REF) %>%
do(left_join(data.frame(NO_REF = .$NO_REF[1], DATE = seq(min(.$DATE)+1, max(.$DATE)+1, by="1 month")-1), ., 
           by=c("NO_REF","DATE"))) %>%  mutate(DATE_X=na.locf(DATE_X, na.rm=FALSE),
             DIV=na.locf(DIV, na.rm=FALSE), KAP=na.approx(KAP)) %>% collect()

在下表中，蓝色行不应位于最终结果中。

预期结果：

预先感谢您的帮助！