R 查找变量的唯一值大于一个的分组变量

在下面的DATA中，我想知道如何找到唯一study_id，对于哪个变量scale具有多个唯一值？

预期答案应该是Li(Li的scale有other和MBTI).但我想知道如何通过BASE或dplyr代码找到它？

m="
study_id   year es_id       r     se     n pub_type  context  ed_setting  age_grp L1    L2    prof  scale outcome
Dreyer     1992   130  0      0.0574   305 DocDisse~ Foreign~ CollegeUni~ Adult   Afri~ Engl~ NA    Other Listen~
Dreyer     1992   131  0.04   0.0574   305 DocDisse~ Foreign~ CollegeUni~ Adult   Afri~ Engl~ NA    Other Writing
Dreyer     1992   132 -0.03   0.0574   305 DocDisse~ Foreign~ CollegeUni~ Adult   Afri~ Engl~ NA    Other Reading
Dreyer     1992   133  0      0.0574   305 DocDisse~ Foreign~ CollegeUni~ Adult   Afri~ Engl~ NA    Other Overall
Ghapanchi  2011    89  0.31   0.0806   141 JournalA~ Foreign~ CollegeUni~ Adult   Pers~ Engl~ NA    Other Overall
Hassan     2001   177  0.25   0.117     71 NA        Foreign~ CollegeUni~ NA      Arab~ Engl~ NA    Other Speaki~
Kralova    2012   137  0.0252 0.117     75 JournalA~ Foreign~ CollegeUni~ Adult   Slov~ Engl~ Inte~ Other Speaki~
Li         2009    55 -0.04   0.132     59 JournalA~ Foreign~ CollegeUni~ Adult   Chin~ Engl~ NA    Other Grammar
Li         2009    56  0.355  0.124     59 JournalA~ Foreign~ CollegeUni~ Adult   Chin~ Engl~ NA    Other Pragma~
Li         2003    57  0.039  0.0735   187 JournalA~ Foreign~ CollegeUni~ Multip~ Chin~ Engl~ NA    MBTI  Overall
"

DATA <- read.table(text = m, h=T)