How Many Characters Are There Where The Radical Is The Same As The Main Component?
A quick perusal of my database gives me

u-cjk/6299      抙  ⿰扌手
u-cjk/726a      牪  ⿰牜牛
u-cjk/72be      犾  ⿰犭犬
u-cjk/73cf      珏  ⿰王玉
u-cjk/8aa9      誩  ⿰言言
u-cjk/8e00      踀  ⿰足
u-cjk-xa/3908   㤈  ⿰忄心
u-cjk-xb/201a7    ⿰亻人
u-cjk-xb/23c99    ⿰氵水
u-cjk/9342      鍂  ⿰金金


(considering only Unicode code points). I tested against the RegEx 氵水|礻示|⺬示|釒金|钅金|王玉|忄心|竹|讠言|言言|冫氷|扌手|足|⻗雨|爫爪|亻人|火灬|犭犬|牜牛|飠食|食|饣食, which may be incomplete. 

In related news, a search for 水水|示示|金金|王王|玉玉|心心|竹竹|手手|足足|雨雨|爪爪|人人|火火|犬犬|牛牛|食食 brings up, amongst others, these:

u-cjk/4ece      从  ⿰人人
u-cjk/6c9d      沝  ⿰水水
u-cjk/708f      炏  ⿰火火
u-cjk/73a8      玨  ⿰王王
u-cjk/7958      祘  ⿰示示
u-cjk-xa/3e5c   㹜  ⿰犬犬
u-cjk-xb/225f0    ⿰心心
u-cjk-xb/22a92    ⿰手手
u-cjk-xb/24519    ⿱爪⿰爪爪
u-cjk-xb/24934    ⿰玉玉
u-cjk-xb/25d12    ⿱竹⿰竹竹


some of which may violate your expectations about how to correctly write characters, as the usual positional variants are not used in these forms.