Tanaka sentence: アマゾン川は延々と北ブラジルを流れている。 The Amazon River meanders through northern Brazil.
延々と流れ	5309
蜿蜒と流れ	0
えんえんと流れ	195
---
延々と連なる	1059
蜿蜒と連なる	0
えんえんと連なる	31
---
延々長蛇	        315
蜿蜒長蛇	        40
えんえん長蛇	21

Despite what the kokugos day, 延々 is also used for sense 2 (and is much more common than 蜿蜒).

@@ -14,0 +15 @@
+<ke_inf>&rK;</ke_inf>
@@ -17,0 +19 @@
+<ke_inf>&rK;</ke_inf>
@@ -20,0 +23,5 @@
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>蜒々</keb>
+<ke_inf>&sK;</ke_inf>
@@ -28,0 +36,3 @@
+<stagk>延々</stagk>
+<stagk>延延</stagk>
+<pos>&adv-to;</pos>
@@ -30,2 +39,0 @@
-<pos>&adv-to;</pos>
-<gloss>forever</gloss>
@@ -33 +41,2 @@
-<gloss>on and on</gloss>
+<gloss>continuously</gloss>
+<gloss>(going) on and on</gloss>
@@ -39 +47,0 @@
-<gloss>wandering</gloss>
@@ -43 +50,0 @@
-<gloss>zigzagging</gloss>

It looks like this needs to be split.
GG5 is my only reference which merges these forms.
Going by the kokugos, it looks like 延延 applies strictly to our sense [1] and 蜿蜒 is sense [2].

蜿蜿 and 蜒蜒 (distinct from 蜿蜒) are both mentioned in meikyo and iwanami.

Google N-gram Corpus Counts
╭─ーーーー─┬─────────┬───────╮
│ 延々　　 │ 351,921 │ 83.5% │
│ 延延　　 │   2,099 │  0.5% │
│ 蜿蜒　　 │   1,573 │  0.4% │
│ 蜿々　　 │     131 │  0.0% │
│ 蜿蜿　　 │     157 │  0.0% │
│ 蜒々　　 │      88 │  0.0% │ 🡠 not currently in JMdict
│ 蜒蜒　　 │       0 │  0.0% │ 🡠 not currently in JMdict
│ えんえん │  65,743 │ 15.6% │
╰─ーーーー─┴─────────┴───────╯