covid19 %>%
  group_by(Date, Continent) %>%
  summarize(across(where(is.numeric), sum))

结果

Date       Continent Positives Something_else
  <chr>      <chr>         <int>          <int>
1 2020-02-05 Europe           15              6
2 2020-02-06 Europe           20             15

如果只有一列数据要求和，可以使用count(wt = ...)快捷方式：

covid19 %>% 
  count(Date, Continent, wt = Positives, name = "Positives")

#        Date Continent Positives
#1 2020-02-05    Europe        15
#2 2020-02-06    Europe        20

示例数据与另一列数字数据相加（可能由OP问题“是否有简单的方法可以将日期相同的所有列相加？”暗示）

covid19 <- data.frame(
  stringsAsFactors = FALSE,
              Date = c("2020-02-05","2020-02-05", "2020-02-05",
                       "2020-02-06","2020-02-06","2020-02-06"), 
                      # NOTE UPDATED DATES VS OP
         Continent = c("Europe","Europe","Europe",
                       "Europe","Europe","Europe"),
           Country = c("United Kingdom","Poland",
                       "Sweden","United Kingdom","Poland","Sweden"),
         Positives = c(10L, 5L, 0L, 12L, 7L, 1L),
         Something_else = 1:6
)

以R为基：

aggregate(Positives~Date+Continent,covid19, sum)

        Date Continent Positives
1 2020-02-05    Europe        15
2 2020-02-06    Europe        20